Those are the two "usable" sizes out of a 20X24 sheet. If you cut it to 6 1/4" you still only get 9 squares out of a sheet so I guess they do it to avoid a small strip on one of the sides. Don't really buy into the perfgel idea that much, we stock **** near ever color of Rosco you could want so we custom cut for each show so buying a certain size would be limiting.
Understood. However, there are many theaters and corporate productions that have way too little time/labor to hand cut dozens of sheets of gel to fit conventional lighting fixtures. As a free service to remedy these situations, Apollo offers the 145 colors of ApolloGel (in either 20x24 or 24x24 inch sheets for maximum yield) Perfed to order.
With growing annual gel sales, we see Perfgel as providing a solution to a very common problem- too little time.
The 6.625 inch cut is not aimed at the S4 sized color frame as much as the popular 7.5 inch color frame. There are PARs that have 7.5 ich color frames with 6.25 inch apertures. This creates a problem with the gel not quite filling the opening and still having enough material to hold it within the frame.
This cut can be used for conventional 6.25 use, as with S4 fixtures, by simply folding an edge over.
The main advantage is to allow many color frames/fixtures to use the same size Perfed Gel, lessening required sheets on the shelf.