Tired legs = lower end. Fresher=higher end. Really if you think about it, before you do a new FTP test you should be capable of riding at the higher end of each zone. Your body has (hopefully) changed, but you are still training in the zones you set off your last test. Last week's FTP interval could be next week's sweet spot interval. So technically, riding at the low end isn't actually doing the work you want, but also remember that these zones are not an exact science. Just a system to structure your work. Heck a poor night's sleep or bad hydration could have a bigger effect on your body's ability to hit a specified zone than you realize.

You are probably over-thinking this. Just try to stick to your plan and don't stress too much.