A loose cushion has to fit its seat exactly, and 'exactly' does not mean 'the same size'.
A cushion cut to the precise dimensions of the seat opening will look loose the moment it is filled, because filling makes a cushion barrel outward at the sides and lose length. So the cover is cut oversized by a calculated amount - more for a soft filling that will compress, less for a firm one that will not - so that once filled it sits into the opening under slight tension, touching the frame all the way round.
Our upholsterers work out that allowance from the filling before the cover is cut, not after, and they make the cushion before the chair is finished so it can be tried in the seat and adjusted.
The result is a cushion that stays where it is put rather than sliding forward every time someone gets up - which is the single most common complaint about loose-cushion chairs, and it is always a cutting problem rather than a cushion problem.