Because 1) the various metrics inspired by TSS were developed by different people using somewhat different approaches, and 2) even if The International Committee for the Homologization of TSS (of which I was the self-appointed sole member) had had its way*, the scores would still not necessarily be additive, simply because each sport has its own unique demands.

*I'm not joking, i.e., a couple of years ago I proposed developing a universal stress score (USS) to replace all the various TSSs, and did a fair bit of the research legwork. The powers-that-be got cold feet, though, so I shelved the idea.


I haven't seen the Strava chart to which you refer, but in general their approach is simply to rename things I've come up with (e.g., "suffer score" instead of TSS) to avoid TrainingPeaks' copyrights, then do a half-arsed job of implementing them. I therefore certainly wouldn't hold them up as an example of cutting-edge stuff.

As for WKO4, at the present time your choices are to either 1) not add TSSs from different sports together (what I've already recommended), or 2) add them as you desire (e.g., TSS + rTSS but not +sTSS). How they might interact is really not known, so any sort of weighting would simply be a WAG.