Short Engine Variations

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Chessbovine

I've been curious, usually the engine calculates 15 moves ahead, but sometimes it's just one move like in this example r1b2k2/pppp3B/2n2p1p/2b1p3/4P2P/6n1/PPPP4/RNB2K2 w - - 0 6

It's obvious if there is only one legal move when a king is in check, but why is the best move for white here only displayed as only one move ahead and also showing only two variations when it is set up to show three?