That's completely understandable in this case of programming in Prolog.
Prolog is a declarative language.
You declare the rules, and the system figures out a result that matches those rules.
The problem is that this basically doesn't work. So a Prolog programmer has to write the "declarative" rules in a procedural order so that the run-time system doesn't have to try every possible combination to find (or fail to find) a matching result.
The proper ordering of declarations can be quite subtle. With a modestly complex program you can make a seemingly unimportant change in the order of the declarations and have the runtime go from a second to a week.
In this case the professor didn't (couldn't) know how long a Prolog program to solve this problem should take. He just assumed that you had found a more efficient ordering for the declarations. He might even have thought it was luck rather than deep insight that your program was faster than his. But you have to a decent understanding of the limits of Prolog to get a complex program to complete in a reasonable time, so you had to be good before you could get that lucky.
If you couldn't already tell, I have a low opinion of Prolog and declarative languages. They are "parlor tricks". Much like computer 'Life' and neural networks, simple programs can produce unexpected seemingly-sophisticated results. But they don't get better from those initial results. To compute the answers or results you want in a reasonable time, you end up with at least the complexity of writing things in the traditional explicitly procedural method.
The promoters of declarative language typically don't mention that you end up writing rules in an explicitly procedural order if you want the program to work. If you press them on the issue, they then say "well, OK, it's not actually any easier to write, but it's easier to verify that you've correctly specified the desired result." But if you have to carefully shuffle declarations around, and even then some results unpredictably take centuries to compute, pretty much no one cares.