But I think maybe thats just some kind of rounding error.
For what it's worth: There are not supposed to be any rounding errors in Bitcoin. It's exact, up to the last digit. If it doesn't add up, something is wrong.
Given that it appears the OPs wallet is not synced, the changes to the balance were most likely a result of the wallet syncing in the background and adjusting the wallet balance as new transactions were found and processed.