Number of possible hands in last two rounds of Texas hold-em
An explanation of how these numbers were derived is available in Johanson, Michael. (2013). Measuring the Size of Large No-Limit Poker Games.
Essentially those are the number of card combinations from one player’s point of view in a heads-up limit Texas hold’em game after merging strategically identical combinations.
How to count different card combinations with isomorphism? over on the Mathematics stack has more information on the combinatorics used to losslessly merge isomorphic card combinations.