Sometimes it is down to rounding or honesty issues: checked the detailed specs. When I was last shopping for monitors several advertised as 24" officially had a 23.6" display area when you looked at the detailed specifications. The difference could have been down to simple rounding by marketing and/or the retailers or perhaps there was a bit of visible panel each side with no pixels (that some were counting as they were quoting panel area and others were not as they were quoting actual display area).
If the display sizes genuinely differ by that much then the 22" has about 2.3% larger display area so will be better assuming all other things are equal. If the smaller screen is a better quality panel by other measures then that may be more valuable than the larger area.
Monitors labeled 21.5" usually have a resolution of 1920x1080, while 22" monitors usually have a resolution of 1680x1050.
I suspect that is someone extrapolating a very small set of data points to produce a view of the whole market that is inaccurate.
On a semi related note, those two resolutions are different aspect ratios. 1920x1080 is 16:9 and 1680x1050 is 16:10. The 16:10 equivalent of 1080p is 1920x1200 - about 11% more pixels down. A 22" diagonal at 16:9 is 19.17x10.78 (~206.7 square inches) and at 19:10 is 18.65x11.66 (so less wide but taller, with a slightly larger area at ~217.5"2). Coders and designers tend to prefer a 16:10 screen, though they are generally more expensive due to the economies of scale as far less 16:10 panels are made and sold (because almost all TVs and most monitors are 16:9), and general home users prefer 16:9 as video content is more likely to target that ratio so they see less unused screen space (due to letter-boxing) when playing movies.