Membership plugin/Subscription Expire

If someone signs up to my monthly subscription membership, pays say $20 and then decides they do not want to the membership:

When they hit unsubscribe, and go via the Paypal system to cancel the payment plan, once they re-visist my site and login it says:

You are currently on the subscription : Subscription Membership

Your membership has been cancelled and will expire on : 27th May 2011
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They have just paid for a 30 day subcription, even though they have cancelled, they should still have 30 days left to access teh content, why does this say the membership will expire on the same day they cancelled?

Thanks