The MHz value is determined by the manufacturer, but despite this, the card can boost slightly higher as well.
It's sort of GPU self overclocking. The Card boosts to way beyond advertised clock due to available margins. It's normal. Think of GPU Boost like Intel's turbo boost.
It kicks in automatically when needed as long the card is not on power or thermal limit. But unlike Intel's turbo boost, it can't be disabled. The "boost" relates to GPUBoost 4/3.0 which will essentially overclock a card beyond the base/max, assuming various criteria are met (thermal, voltage etc).
Most of the time, cards will boost beyond the stated boost clock, but it is dependent on thermals and other factors. What you're seeing is not abnormal, and is relatively common, IMO.