We’ve covered this before but I’ll say it again. You set your arms at 90d. When you pump aggressively, it’s impossible to keep the angle and it becomes tighter to the front and more open to the back, however the elastic return response created by trying to hold the 90 does much of the work initiating the return.
If you deliberately open out, that elastic response is lost and you end up doing more work more slowly.
That’s the difference between what you see and what’s really going on at the top level.