Why does BrokeredMessage.RenewLock() only renew th

I've got a BrokeredMessageContext class which uses a Timer to periodically check and renew the lock on a BrokeredMessage instance in case the process that handles this message runs for longer than expected. It renews the lock by calling the RenewLock() method on the BrokeredMessage instance.

I expected this call to give me a new lock with the same timeout as the original lock (MSDN states that "You can renew locks for the same duration as the entity lock timeout, and there is no maximum duration for a lock renewal."), but when debugging it appears that the lock timeout is increased by an 'arbitrary' 10-15 seconds. I've set a watch on the BrokeredMessage instance and I can see that the LockedUntilUtc propery has 10-15 seconds added to it every time I call RenewLock().

Does anyone know why this is the case? Can anything be done to renew the lock for longer?

EDIT:

Mike's answer, below, is correct. I discovered that I was, in fact, trying to renew the lock every ten seconds right from the start even though my code was intended to renew the lock now earlier than twenty seconds prior to the lock expiring. It all came down to a time comparison issue and the fact that the time on my machine was wrong (it was ahead by nearly a minute). D'oh!

When you call RenewLock it resets the time the message is locked by the LockDuration set on the Queue or Subscription the message is pulled from. If your class that is performing the renew has a timer that is firing every 10-15 seconds then the behavior you are seeing is correct.

Example: I have a queue with a lock duration of 1 minute (the default).
I pull the message at 1:20 AM UTC, so the LockedUntilUtc should read 1:21 AM UTC. If 10 seconds into the processing I call Renew lock the call fires at 1:20:10 AM, so the LockedUntilUtc would become 1:21.10 AM.

It added the lock duration value to the current time at the server, not to the previous LockedUntilUtc value. This extends your period of the lock on the message.

This answer assumes you are firing the trigger on the Timer often and not waiting to get close to the actual lock timeout. If you wanted to be more precise you could set the timer to fire about 10-20 seconds before the lock is set to expire and then do the renew lock.