When I call him to discuss potential shidduchim in the yeshiva for girls my wife knows, he does not look at age or anything the letter encourages. If he believes the appropriate match for this 20 yr old girl is a 25 yr old bachur, he says so, the same way if he felt a 20 yr old was the most suitable match.
Why did your R”Y sign it, if he doesn’t believe in doing what he signed asking people to do?
Here’s how I see the issue. Although it’s clear that closing the age gap would alleviate the numbers disparity, that doesn’t necessarily mean that a closer in age shidduch is better for the individuals involved.
If a bochur asks his R”Y for advice as to whom to marry, and the R”Y feels that he needs a younger girl (personality, maturity, etc.), then he can’t very well recommend for this bochur that he marries an older girl – it would be lifnei iver.
Similarly, if a girl (or her parents) would ask for advice (even from one of the signatories), it would seem that the best advice for her would be to try to get married as soon as she’s ready, and not wait just to help another girl.
So although there may seem to be a contradiction, there really isn’t one. On a societal basis, closer-in-age shidduchim would be beneficial, but on an individual basis, it might or might not be.
A R”Y signing in favor of the ideas of NASI is agreeing to the societal benefit, but not necessarily that it’s to the benefit of a particular individual.