I think it may be because of the size of the clips it may use to secure it, if it is an undermount sink. When I was researching my sink/ cabinet options, I found that many sinks required at least 2 or even 3 inches of space on each side for install.

I think you may well be able to install the sink in a smaller cabinet than stated, but check with your installer to be sure. Some reviews of the sink (if it is listed on Amazon) may give you an idea what others have done. We put a 23" wide sink into a 24" cabinet.

This is good advice. Yes, check with your installer. The sides of the sink base may need to be cut to accept the sink, and he might not be comfortable in doing so.

I also meant to mention that cabinet sizes....in this case, 21"....refer to the total exterior size of the cabinet. Each cabinet side is usually 5/8" thick. So, for a 21" cabinet, you would be left with 19 3/4" of space to install your sink.

You can shoehorn an 18 1/2" sink into a 21" cabinet. Sink clips are obsolete these days. Use a Hercules Universal Sink Harness or a Sink Strap.

If this is a top mount sink it's a nonissue. If this is an undermount sink, the key word is shoehorn. Construction workers seem to have large hands and they have to get their hands (and the wrench for the faucet) up between the sink and the walls of the cabinet to attach the sink, sink strap and to add the faucet.