Robinhood margin calculated wrong?

You have 100k equity. You borrow 100k margin, so now you have a total of 200k to play with. You buy 200k stock. It goes up 10%, landing at a new total of 220k.

Based on my understanding it seems like your equity should now be 120k, and your margin 100k.

But it seems like Robinhood is calculating/charging interest as if it were 110k equity, and 110k margin.

The problem is that I didn't borrow extra margin to pay for gains. I borrowed 100k. So why am I being charged for gains like this? Is my understanding incorrect? It seems like I should have increased only my equity, and should actually be able to borrow another 20k of margin thanks to my increased equity.

I'm mostly basing this off the account summary page that shows Total market value, Equity, and Margin Used.

Safe harbor: It's entirely possible I've made a mistake here, as I haven't been watching it super closely. I'd be quite happy if I'm just misinterpreting my own numbers. What I'm asking I guess, is, is my interpretation of how it's supposed to be correct?