Simple Compound Interest Calculation Question + [bonus question]

[NickNick]

On a similar note, this is confusing me:

Initial Investment: $1000
Yearly addition: $1000 for 20 years
Interest Rate: 5%

If interest compounded yearly = $37,372.55
If interest compounded monthly = $37,108.17

http://www.moneychimp.com/calculator...calculator.htm

Why is the final amount less if the interest is compounded more regularly?

Thanks a lot! - Sorry if this is a basic oversight!