In 1:400 scale;
E-170 is 74.75mm
E-175 is 79.125mm
A difference of 4.375mm
E-190 is 90.6mm
E-195 is 96.6mm
A difference of 6mm
Bearing in mind that it costs money to make moulds from scratch and that as there are not a large number of E-175 or E-195 customers a manufacturer
may not consider it to be cost effective to cater for the E-175/195s as separate moulds.
Indeed, manufacturers have taken this averaging approach in the past and the collectorate has apparently lived with it. Examples: Comet 4/4B/4C, Convair 240/340/440 (non-radar), and likely the DC-6/6B for starters.
Sliding mould technology has been mentioned and while this would solve all the problems in a stroke it is my understanding that a sliding mould is
considerably more expensive than a fixed one.
Whether or not fudging the length differences outlined above is acceptable to the collectorate or not is another matter. I personally think it is 'stretching' things too far. Unfortunately the manufacturers though are not in the game of throwing money away. The fact that there are fewer liveries available for the E-175 & E-195 may mean that they are not willing to take the risk of dedicated moulds initially and may consider 'fudging' things.
The E-170 and E-190 are a much safer starting bet and I expect we will see them from Gemini in the not too distant future.
Nice little sarcastic remark about the A320/321 BTW.
Nothing personal, but did I offend your precious national pride by 'picking' on the E-Jets?