Missing 'R' numbers?

[traildriver 45]

Why are there large gaps in 'R- numbers" when new cars are ordered now as compared to years ago when they were almost numbered consecutively?

[SubwayGuy 2,321]

http://www.thejoekorner.com/cars/rroster.htm

[traildriver 45]

Okay, thanks for that link. Seems like there would be some better way to designate car types than linking them with orders for any and everything. I notice there are many blanks still.