I can't speak on behalf of watermelon, but I have grown cucumbers on the ground and the same variety on a trellis. Days to harvest were the same.

I can say that some of my crops are not ready to pick according to what the seed package "estimates". My beets always take longer than what the package says. Same thing on my carrots.

Partly it depend on the number of heat units accumulated in your area, amount of sunshine, water ect. The estimated ready date on the seed package is just that, an estimate.

My brother grows cantaloupes and watermelons and if lucky honeydew melons. He always laughs at what the estimated ready dates say as his melons take a lot longer before they are ready to pick.

That helps a lot. Thanks!