the single rate is always higher - basically, they look at it that, if there are two people in a house, they will share the expense of things like gas, electricity and water. if you're single, you'll pay twice as much for these things. so, only one of you will get the payment, and it will be paid at a lesser rate than what a single parent on carers would get (as they have to foot the bills on a single income)

i guess they look at it that, if you're single, you get 500 for everything. if you're partnered, you won't have twice the expense, so you get 800 ish between you

i so hope that makes sense - very hard to put it in words when you can't confirm understanding as you go lol