It seems to be proportional to the open positions I have. I haven't observed long enough to see the rules. I hope someone else may shed some light on this. The dollar amount could be large if you have large positions.
I doubt that the balance is 'wrong'. You'll need to check what margins your account is set to and what other restrictions, if any, OEC may have on your positions. Get on the chat and ask. That's what it's there for. Email could take longer whereas the chat can happen right there.
It's always dead on for me. If you are holding T-Bills in your account (a nice OEC option btw), you only get credited for 95% of t-bill value until it matures.
Hold the phone! I had no idea they allowed that. Can they facilitate the transactions or do you have to deliver them in?