git reset for index restoration?

If I have a git repository with a clean working tree, and I delete the index, then I can use git reset (with no arguments) to recreate it. However, when I do recreate it, it doesn't come back the same. I have not analyzed this in detail, but the effect is that commands like git status take much longer because they must read objects out of a pack file. In other words, the index seems to not realize that the index (or at least most of it) represents the same state as HEAD. If I do git reset --hard, the index is restored to the original state (it's byte-for-byte identical), and the pack file is no longer read.

Before I try to dig in to why this should be so, does anyone happen to know off the top of their head? Does this constitute a bug in git, or a bug in my understanding of git?