It might have done just the same if it had been set to indexing just the same. Sounds to me like the lower limit stop wasn't set down far enough.

When setting RD's, I usually like to adjust the derailer no further to the left then necessary - if the derailer in question will shift into and retain the low gear (even when tourqed hard) whilst sitting a fraction of a mm off to the right of the cog, I will leave it there.

Otherwise, I will center it, but under no circumstances allow the derailer to sit even minutely off to the left to aid shifting. If the cage appears to have an inward swing to it, I'll also check the dropout and derailer to check for bend damage. If derailer related, it gets replaced with a straight RD - pie-plate or not.

-Kurt