I believe what you've got there is a standard round-off error. To calculate 11^25 with the perfect accuracy you want would require at a minimum an 87-bit mantissa, which goes well beyond what the double-precision floating point Mathstudio is likely using can hope to provide.
What you need is either an arbitrary-precision math package (Which MathStudio is NOT), or a dedicated function for calculating modular powers, such as this:
Frankly, I find it sort of surprising that MathStudio doesn't have a modular power function built in. Am I missing something, perhaps?