By documentation, the parameter of DIM must be a positive numeric constant. DimQty^3 is a constant expression rather than a constant. It would be correct to report an error as in the case of no OVER. The compiler does not allow not-local variables with not-constant size - it reports the error
Variable-size must be constant
in this case on attempt of allocation that variable. Because variables with the OVER attribute are not allocating, not-constant parameter of DIM as in your example remains not detected and reported.
The compiler computes constant numeric expressions if all parameters are either Integer or Real numbers (or EQUATEs of these types) and if they contain only following operators:
Addition +
Subtraction -
Multiplication *
Division /
Modulus %
The Modulus operator with Real type parameters also does not allow calculate the expression at compile time. If the compiler can compute the constant expression, it uses its result.