the series are not continuous ex. after gtx 690 it is not gtx 720
eah series is differentiated by a better architecture/manufacturing process/improvement
ex- 7xx - kepler 9xx- maxwell(architecture)
gt 720 is the wakest card while the gtx 780ti is the strongest card in the series
likewise gt 620 is the weakest while gtx 690 is the strongest
usuall the performance improvement is like this
gtx 9(n-1)0=gtx 7
0 where n=1-8
ex - gtx 970 is as powerful as the gtx 780
NOT- this is not official but just general
hope you can understand!