Ah, yeah, so what it's showing is the average bonus that crit provides, not the total for a single cast. There's probably a better way to explain it.
Assume a base heal of 100, and crit at 20%. It should crit once out of every 5 casts:
100 + 100 + 100 + 100 + 150 = 550 total healing
550 / 5 = 110 average per heal
So... that 20% crit increased your average heal by 10 points. That's what the chart is trying to show. If you increase your crit chance by 50% (from the talent) it'd raise it to 30%. So... if we take 10 casts it should crit 3 times...
150 + 150 + 150 + 100 + 100..... = 1150
1150 / 10 = 115
So that 30% crit increased your average heal by 15, which is exactly a 50% increase from what your crit previously did(untalented). Whenever you check that box for Improved Regrowth, it increases the crit bonus by 50%. One of the reasons the numbers are so low for regrowth is that crit doesn't affect the HOT (as far as I know, I didn't actually test that now that I think about it). The crit ends up only affecting the direct heal, which is roughly half of the healing the spell does. I made the chart this way because I wanted to be able to compare what 1 bonus healing does versus 1% bonus crit.