It does not seem right to say max output is 3dBm if the actual output is less, but there is no tolerance on the numbers anyway.
Exactly. It is mathematically correct to round say 3.6 to 4, regardless of the units or scale.
I recall a time when hard drives became available for home PC users (just before '386 era) in my part of the world, and the manufacturers' marketers used -- and still use -- "megabyte" for 10
6 bytes, even though they knew quite well that the established value for a megabyte was 2
20 = 1048576 bytes, or 4.8576% more.
Mathematically, the marketers were right! That's why we have
mebibyte instead. (I don't think I've ever used the full word out loud, but I do use kiB, MiB, GiB, and TiB for 2
10, 2
20, 2
30, and 2
40 bytes, respectively, to minimise confusion. Some still refuse, calling 2
30 bytes a gigabyte.)
So, while it does not
seem right, it is still the mathematically correct thing to do. Rounding from halfway could go either way, though.
If the specified figures are too vague, there is usually a good reason for it. Perhaps the quality varies, or perhaps the manufacturer does not even know. Sometimes it is so that the marketers can round it to a bigger figure; similar to but opposite for pricing items at e.g. 9.99.