Hi Nora,
Yes, guys lie about their height in this way on dating sites.
If their height is relatively close to the next definitive height, perhaps short by just a few inches, they often add those extra inches to get to that next tier.
This is especially true the closer they get to 6 feet. The average height of most men in the real world is around 5’9″.
Dating sites are often a reflection of society at large.
If you are seeing an unusually large number of dating profiles claiming a height of 5’11”, that goes against the overall height average of most men in the real world. This makes it more likely that several of these men are adding extra inches to their height.