The reason for not being fixed c in cell

Hello everyone,

I have a hexagonal mxene. during the optimization , c lattice constant changes. Assuming I want to set the c to 20, and with the constraint option, I fixed c, but I noticed that after the optimization, the value of c changed to 14.5.  what is the reason?  Why did its value change from 20 to 14.5 despite fixing c?