In geometric terms this can still be an extrapolation, even without going beyond the space defined by the axes. Suppose a weight and width axis. A master at (weight=1), another master at (width=1). If we look at a corner, (weight=1, width=1) we would see the effects of both masters superimposed.
Variable fonts allow (and designers expect) to be able to add another master at (weight=1, width=1) so that extrapolation can be adjusted. Depending on the design of the typeface and the structure of the designspace some axis combinations might need masters, but others might not. The designer can decide that some combinations do not make typographic sense, and that it is not necessary to define what it should look like.