Why is there no clear fair answer? When both party's primaries are held at the same time, voters know who all the candiates are from each party and can make a fully informed decision as to which ballot they want to influence.
In the first primary, Graham was essentially unopposed; he was a sure thing. So many R-leaning voters may have decided to vote in the D primary, perhaps to tip the race to a weaker candidate.
Should those voters now be locked out of voting in the special R-only primary?