EXACTLY. But from what I have been reading it doesn't seem that is what was done. Because they broke the 15,000 imaginary threshold, neither Romney or Paul had that done to their petitions, but because neither Gingrich or Perry didn't make that threshold (although meeting the state statute numbers) they were subjected to verification - which is not required by statute for national office.
But supposing Romney turned in 17,286 and Paul turned in 18,884 and the general disqualified signature rate was, say, 6%, then not sampling the signatures may have been statistically justified.
Further, if Perry turned in 10,380 and Gingrich turned in 10,651, then it would most certainly be prudent to examine their petitions since they are within the statistical disqualification rate range.
I really can't say from the information given. Thus, if the Virginia GOP can show that standard statistical sampling techniques were used and no political bias was involved, then there is ample justification for tossing the suit out.
However, another report is saying the Virginia GOP is requiring voters to sign a loyalty pledge to support the eventual GOP nominee. If so, that would certainly weaken their case since there is no precedent for doing it before.