Differences in the sha hashes are a result of infrequent updates to the i2pupdate.zip being served from http://skank.i2p/i2pupdate.zip ... occasionally a new update will appear, either to add new features (in the recent case, more complete translations) or fix bugs. To identify exactly which revision you're running, you can view the hashes on 127.0.0.1:7657/jars.

In addition, for some users a new release will revert them to vanilla I2P, so updating the I2P+ zip available on skank.i2p will sometimes fix this if it's date stamped after the update to vanilla.

As for "corrupt" downloads, if you're in the middle of an update when a new version is uploaded, your download will become corrupted, in which case re-downloading the update should fix. It shouldn't happen for release updates very often, slightly more chance when updating from the /dev/ path as that's sometimes updated several times a day.