Those are registered numbers. Tourism from the nineties until Obama, especially from the US, was completely unregistered. Not only that, but Cuba facilitated tourism scofflaws by making it policy not to stamp certain Western passports. (I was there legally, and I can tell you that even the legal visitors, did not get stamped. At least as an American they didn't. They went the extra mile to cover your tracks.)

Point being, reported tourist numbers from Cuba are completely unreliable. (Really any reported numbers from Cuba are unreliable but that's a whole other story.) They report whatever serves the government's story in Cuba. Full stop.

So what government story prompted the gradual growth of reported tourists?

I looked at the data a bit more in an attempt to find sources skeptical of the reported tourists, and I couldn't find anything. I did find the reported tourism revenue though, and though it still seems like far more tourists are visiting, the expenses per person has dropped a fair amount leading to the % of GNP from tourism dropping since 98.

This would explain why the economy doesn't seem to be benefitting despite the high rise in tourism.