That's because corvettes are newer ships and they would be assigned first to the most important sistems (Beta Aquilae and Delta Serpentis) so there wouldnt be a high number in the possesion of the NTF.
That's not necessary true.In the Freedom shipyards in Polaris Bosch made the Iceni,a "super-corvette".Why they shouldn't create Deimos too?
I don't know if someone talked about this before,but NTF ships with <bad> names,like the NTC Refute and the NTD Repulse may be NTF constructed ships(or ships under construction then captured by the NTF,so the rebels have changed their names).The rebels controlled 3 systems,maybe they have constructed one or two Orions,up to three Deimos and even more Aeolus(the NTF has too many Aeolus cruisers for its dimensions,we know that there are 24 of them,but after seeing the Mylae,the Hellespont,the Saharan,the Liberty,the Epigoni,and considering that other cruisers with non-specificated class can be Aeolus...well..)or Leviathans(and,ok,Fenris too).