They do it to give you a verification number.
The number that appears on the withdrawal from your account is your verification number, which must be entered at paypal before your account can be seriously used.
Bank statement list:
Paypal #294489 $2.50
When you next sign in to paypal, they will ask you for the verification no#, which is 294489.
They give you paypal credit for what they took out of your account.
So if they took $2.50, and you pay $30 for a paypal purchase, they will only take $27.50 for the first payment.
It is in the conditions when you sign up.
hint Read the conditions thoroughly before you sign up.