This is because the winner was playing in USD. Had they been playing in GBP, it would have reset to $1 million.
5,556,736 USD is roughly 2,786,030 GBP. (according to Google,
)
Subtract 2,786,030 ($5,556,736) from 5,556,736 (since Microgaming jackpot values are stored in the bank in GBP) and you get 2,770,706 -- roughly the current jackpot size.
When someone wins a Microgaming jackpot, they win the numeric amount shown, in whatever currency they are playing in. This can be USD, GBP, Euro, or South African Rand.
The first-ever Mega Moolah jackpot (Nov. 16, 2007) paid out 4.3 million
Rand, which was equal to about 313,000 GBP at the time. As a result, the first-ever Mega Moolah jackpot reset itself to just under /$//R 4 million!
I hope this makes sense.