To elaborate on what Oli said succinctly:
Michael’s objective was to be the biggest legal gaming mogul in the Western Hemisphere—at that time, Nevada and Havana. I’m guessing that Roth mentioned to him, early on in their negotiations, that he’d give some of his Havana empire to others for the reason he stated later at his birthday party: “to make sure things go smooth at home." Michael went along because he had no choice, and, probably, because he figured that, once Roth stepped down, he’d either buy out the others, or force them out, as he had Klingman.
The Tahoe shooting showed Michael that the only thing he’d get from Roth was a death certificate. So, his objective for the Havana meeting was to string Roth along until he found out who was the traitor in his family, then have Roth killed before Roth had him killed. The high percentages of his holdings that Roth said he’d give to others may have surprised Michael, but at that point the percentages were moot—Roth would “never see the New Year,” as he told Fredo. The grenade killing showed Michael that Batista was soon going to be history. But I don’t think he had given up on Havana at that point. Even if the rebels won, he probably convinced himself that they wouldn’t shut down the casinos—Cuba’s third biggest industry after sugar and minerals.
There was a possible complication. Michael had to leave Havana immediately after Roth’s murder to escape possible arrest or questioning. That’s why he had the private plane waiting. With Roth apparently dead, what was to stop the Lakeville Road Boys, Eddie Levine of Newport, and the Pennino brothers, from moving immediately to claim their shares—and perhaps Michael’s, since he wasn’t there to stop them? Michael had to wait until he thought it was safe to return to Havana. The others may have thought that they didn’t have to wait—it was their big chance to go one-up on Michael.
Batista’s abdication on New Year’s Eve was a stroke of good luck for Michael—nobody could do anything until the dust settled and the rebels took over. As it happened, Castro immediately closed the casinos, and eventually nationalized all American property. By waiting, Michael saved $2 million.
(N.B.: In real life, Castro reopened the casinos after a few weeks because the hotel and casino workers’ unions pressured him. He hired Joe Louis as official greeter at the casinos, and ran newspaper ads in the US explaining that his differences were with the US Government, not American citizens, whom he loved. No one believed him, and Americans stayed away. The casinos and brothels were shuttered permanently. As a result of Cuba no longer being a prime US vacation paradise, other countries in the Caribbean and the northern rim of South America stepped in and legalized gambling.)
