A professor for my one course didn't calculate an attendance and participation grade into my final grade but it was worth 20 percent in the syllabus.
I would argue that an instructor has the right to deviate from the syllabus's method if it does not unfairly penalize any student.
I had this happen recently when I told a class that the weighting of the final exam was different from how it was calculated on the syllabus. Because the change benefited everyone in the class, except for one student who was right in the middle of a grade range (and was only disadvantaged by about 1 point out of 1000), and therefore was not hurt by the change, I kept the more beneficial weighting.
A syllabus is often considered to be a kind of contract, which the instructor would be violating if they changed the terms without notifying the students. But in the case you're describing it's more likely either you or the instructor made an error in calculating your grade. It's certainly reasonable to ask for an explanation of exactly how your grade was determined.