That is the characteristic of English-education in China, for we have been taking British English as the standard one. Traditionally, "less than" is used to compare uncountable nouns, whereas "fewer than" to compare countable nouns. Yet in spoken English, limit seems not to be so strict.

"Fewer than" is used in reference to something counted, "less than" is used in reference to something measured. Less than a gallon, fewer than a dozen. It seems less strict in spoken English only because the terms are commonly misused. It relects poorly on our educational system, not any inability of yours to grasp the concept. By your question, you demonstrate that you understand it perfectly well.