What power meter do you have?

10% difference is normal for me if im not fully trained/adapted to aero position.

When im about to race an ironman, this number is much smaller.

Maybe you are putting less weight on the rear wheel in the aero position and maybe this is causing some unloading of the resistance from the trainer?

I could see this happening with one of those trainers that are L-shaped (rather than A-shaped).

i`ve been using the 4iii for more than 6 months now and it has been performing flawlessly.

- i use the cycleops magtrainer +
- outside if in the drops, power goes down a bit as well but not as dramatic as on the trainer.

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"Pain is NOT temporary,you remember every bit of it"

Yea I can understand your confusion. On a trainer, same gear same cadence should mean same power.

Same gear, same cadence, same trainer, different power? I don't think your power meter has been performing as flawlessly as you think.

 i haven`t noticed anything out of the extraordinary and i calibrate every ride. i`ll observe again later tonight.

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"Pain is NOT temporary,you remember every bit of it"

The reason that road bikes have you in a more upright position is because it is the best position to generate power. More aggressive positions will almost always lower your power output. The aero gain tends to always outweigh the power loss. (Until you become too aggressive) Typically cyclists see their power in a TT position catch up with their road position with more time in the TT position. I just got a TT bike and my FTP is around 20 Watts less on it than my road bike.

Thanks! i guess i was wrong thinking more or the same power output if you go lower via the shorty bars or drops.

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"Pain is NOT temporary,you remember every bit of it"

Sure thing. If you really want to get into it, it has to do with your hip angle on the bike. The more you drop your upper body, the more you close your hip angle which makes it more difficult to put out power. This is why the saddle is further forward on a TT bike. By moving the saddle further forward (relative to the crank/bottom bracket location) you are able to lower your upper body while being able to keep your hip angle more open. (More obtuse) That is why if you're getting fit with clip-on aero-bars, the saddle will get moved forward. (Adapting the road geometry to a more TT style fit)

Neither calibration nor observation will tell you whether your L/R balance is changing when you change your position between sitting and aero. Same gear, same cadence, same trainer, different power means a likely explanation is that you're the thing that's changing.

"shorty" are clip-ons, correct? If so, you may see an initial drop off, but the long term difference should be minimal, especially if you acclimate. I use the "invisible aerobar" position a lot and i may be marginally less powerful in that position. On my real TT bike my power drops ~8% across the board.

Being on the trainer may also make it slightly more difficult

If the original poster is saying what he thinks he is saying, then Rchung is on the money.
If he is not altering settings then this must be so.

But if he is adjusting the load himself and finds that he has to go to a lower load with a higher cadence then Jmjtri has got it.

Otherwise this is fairy dust.

Thanks and yes clip-ons with the shorty extensions so i didn`t have to adjust the saddle, i race 1-2x per year so getting a tri bike as much as i would want one is out of the picture given the state of traffic where i stay and 99% i ride indoors now.

i think they are both correct with regard to the hip angle getting tighter and me changing as well because of the position changing - i`ll compare the next time i ride outdoors.

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"Pain is NOT temporary,you remember every bit of it"

pretty much this -
the delta can get down to very small numbers IF the adaptation is addressed.

Slight differences in muscles groups/areas addressed, slight differences in how the body resists/counters the pressure/torque of pedaling, hip angle, different compression of the lungs.... the list goes on.