Puzzled by contactor

Hello there,

So a couple of weeks ago, I fitted a new condensing unit for a bulk milk tank and fitted a contactor and overload. Unit run fine and has been since fitted, got a call this morning to say it wasn’t running so headed out to farm and found that the coil of the contactor wasn’t pulling it in. Faulty contactor I thought so took another one from the van and started to swap it, when I removed the original one it was stamped 400V for the coil voltage, now can anyone tell me how this even worked in the first place? I’m annoyed at myself for not noticing it but I’m even more confused as to why it worked  in the first place. 
 

thanks 

coil will still have some magnetivity at lower voltage, just not as much so may work