Charged interest after BT

Hey, I've had a very confusing (and frustrating) time with Halifax. I completed a balance transfer on 04/07 to Virgin from my Halifax CC - on 18/07 I was charged interest on the account despite it being empty for 2 weeks. (my statement dates run 18-18 of the month, with payment by 13)
I finally managed to get through to CC services yesterday evening after showing a member of staff in branch my accounts and they said it didn't seem right... and this is where my brain started getting fried, along with my nerves....

I asked why I was charged interest on an account that had been cleared and was told that the interest works a month in arrears - fair enough, but pointed out that that months statement and balance were cleared. I was then told it interest from the month before, so on 18/07 I got charged interest for the 18/05-18/06 balance, though I'd paid interest on 18/06. I was told I had to have cleared a balance 2 months in a row in order not to incur interest the next month and that's how all CCs work. I pointed out that the two other CC I have charge me any interest for the 4 weeks preceding it, not up to 8 weeks before and that when I transferred a Barclaycard to them before, Barclays didn't charge me interest that month as the account was cleared. I spoke to a Halifax manager who then said that I was charged less interest than normal that month because I'd paid off more than the statement amount (obviously, it had been cleared). When I said she wasn't making any sense as it had gone from interest being 6 weeks in arrears to 2 months back to being related to that month's statement....she said she understood. I still don't!

Can anyone help me understand what the hell happened there with Halifax as their method of working out/adding interest isn't something I've experienced from other providers,and I still don't understand their stance... Is this as normal as they said and I've just been 'lucky' with my other cards?