Hi, you've delegated 100SP to steemhunt. In that case that you wanted to delegate additional 100, you had to put 600SP, not 100SP. The delegation system is not adding the number on your total deletation. It just process the final number you input. So, this was how it worked I guess:

1. You delegated 500SP at first time.
2. You decided to delegate 100SP more, so you put 100SP at the second time.
3. But, the delegation system thinks you are changing the delegation amount from 500 -> 100 on @steemhunt.
4. So, in result, you cancelled 400SP of delegation.

Hope it will help for you to understand the Steem delegation system :)