This is normal. The user balances do not correspond 1-1 to wallet balances. Any POS rewards are gained on the hot wallet balance, which is owned by your site.
If you wish, you can calculate roughly what percent on your balance you gain via POS mining daily, then set up a recurring Airdrop, to distribute this percentage daily on that coin to users.
Or you can choose to only pay out a part of the POS rewards and keep the rest. It’s up to you.
If you have any more questions about the plugin I am happy to answer them.