No surprise that Cruz wants Kasich to drop out of the race. The general belief, though I have no hard evidence, is that Kasich draws more votes away from Cruz than from Trump, giving Trump an extra advantage, particularly in the winner-take-all primaries.

But why is Trump also calling for Kasich to drop out? It’s mathematically impossible for Kasich to get a majority of delegates from the primaries. And one would think what’s bad for Cruz is good for Trump.

One possibility is New York, Trump’s home turf. New York’s Republican allocation awards all the statewide at-large delegates and those in each congressional district, fully to a candidate only if that candidate gets at least 50% of the vote, statewide or in each district respectively. Even if Kasich draws more votes away from Cruz he still takes some votes away from Trump and could bring Trump below 50% in the state or some districts.

That seems short sighted on Trump’s part. Kasich won’t drop out for New York and then drop back in and many of the remaining other primaries, especially delegate-rich California, are winner-take-all.

Another reason might be the rule that only candidates who win at least eight states (Trump and Cruz) are eligible to be nominated at the convention. Rules can be changed but only with a majority vote of the delegates. Trump and Cruz combined will surely have a majority of delegates pledged to vote for them, but delegates get chosen in different ways in different states and some of them may vote for a rule change even if they are pledged to vote Trump or Cruz in the first round. If Kasich can pick up a moderate number of delegates in the remaining primaries, that could affect whether a rules change could pass.

Why does Kasich stay in the race? He can only get the nomination if Trump doesn’t win in the first round, and dropping out would increase those odds. Maybe he’s trying to get support to change the eight-state rule. Maybe he wants to be the only establishment candidate still officially in the race at the convention. Maybe he’s trying to get Cruz to offer a cabinet or VP position in exchange for his dropping out. Maybe he’s just too vain to drop out. Even politicians don’t always play strategically.

Lance Fortnow is a professor and chair at the School of Computer Science at the Georgia Tech College of Computing