Pushing a GTX 550 Ti over 1GHz

Been scouring the internet for answers, but I never really found a concrete answer.

Why is it that some GTX 550 Tis can hit speeds like 1001 MHz (3DMark link - though no more than that, from what I've seen), while most others are limited to 999 MHz, no matter what the voltage is set at?

(The core clock can be set to a higher number, and the 'graphics card' tab in GPU-Z reflects that, but the actual running core clock (on the sensors tab) never goes past 999.8 MHz. MSI Kombustor says it's throttling.)

Is it a hard limit imposed by Nvidia (i.e. you just can't)? If so, why the variations? (1001 MHz vs 999 MHz)
Or is the card really going above it's safe operating limits for a moment, then throttling?

Finally, is there any way to circumvent this limit? (Risks of reducing GPU lifespan are obvious, but can other components like the motherboard be damaged if this limit can be circumvented?)