I did antelope (5x10m SS) on Tuesday and completed it without issues.
Then on Thursday I did antelope -4 (4x10m SS) and had trouble getting the last interval done.
What gives? I feel like some days sweetspot work feels manageable while others my HR is through the roof?
Cumulative fatigue.
…and, sleep, rest/recovery time, work stress, nutrition…
Essentially, you need to look at EVERYTHING as a whole to find which of those issues (and likely others) may have influenced the differences.
Tuesday was after a rest day I assume and Thursday was after 2 rides (if in mid volume). As others have said, we’re not machines that can produce the same on any given day, way too many variables involved.
Just realize that you’re not alone and that’s normal. I had a similar experience where a week prior I actually increased the intensity during my last over/under workout, but the following week I was grinding and barely getting through SS.