It is just luck on how well each overclocks... Factory overclocks on aftermarket cards are all pretty close... Here is a list of what one site did for max overclocks
ASUS GTX 980 Ti STRIX 1472 MHz 2070 MHz
Palit GTX 980 Ti JetStream 1515 MHz 2100 MHz
ZOTAC GTX 980 Ti AMP! 1465 MHz 1990 MHz
MSI GTX 980 Ti Gaming 1507 MHz 2040 MHz
EVGA GTX 980 Ti SC+ 1491 MHz 1900 MHz
Gigabyte GTX 980 Ti G1 Gaming 1512 MHz 2100 MHz
GTX 980 Ti 1437 MHz 2070 MHz
GTX Titan X 1298 MHz 2055 MHz
First number is core clock speed second number is memory speed.