Savings account interest - please explain!

[ByebyeKaren]

Hi, I've copied and pasted below. Can anyone explain why the interest would be £136.50 as I calculate it to be £252 based on 7% paid to a balance of £3600?
I'm obviously not understanding. Thanks very much.

Get into a good savings habit
And the future you will say thanks. Put away
between £25 to £300 each month, for a fixed 12 month term, and we'll give you a fixed rate of 7.00% AER/gross p.a.
7.00% AER/Gross p.a. fixed for 12 months
save between £25 and £300 a month, up to £3,600 per year
change the amount of your standing order at any time
^^
Interest example: if you save £300 every month for 12 months and qualify for the 7.00% AER/Gross p.a. interest rate, you'll earn approximately £136.50 interest (gross). Interest is calculated daily and paid 12 months after you opened the account.
AER stands for Annual Equivalent Rate. This shows you what the rate would be if interest were paid and compounded each year. Gross is the rate of interest if interest were paid and not compounded each year.

I haven't done the maths but I assume it would be because you save max of £300 per month so don't get a year's interest on the whole amount just on what is already saved. So the first month you get 7% interest on £300, the second 7% on £600 etc.

Because you don’t have £3600 in the bank account for the full year

It’s because you haven’t got the full £3600 in the account for a whole year. You can only drip feed the money in at £300 a month. The 7% is the annual rate. For example in month 11, you put the final £300 into the account. You’ll then only get a month’s interest on that £300 (ie 1/12 of 7%).