Money confusion again

I know I've been doing a lot of talking about maths the last few days and every time I do, I come up with something different.

For easy maths I'll round the money to the nearest £10 lol

I get £760 a month
Direct debits come to £280
I move £10 a week into B
Shopping is £100 a week.

So, here's todays maths on the assumption that there are 4 weeks in a month:

£10x4=£40
£100x4=£400
£400+£40=£440

Right so far?

£440+£280=£720

Yeah?

£760-£720=£40

So what was I spending that £40 on yesterday?  Why do I have it to spend now but I didn't yesterday?

I give up!  lol