About 5% of the population will grow up to qualify for a “developmental dyscalculia” diagnosis. Children with developmental dyscalculia, relative to their overall intelligence, show significant delays in the area of mathematics.
Many of these children, when they were six months old, wouldn't have been surprised over seeing five puppets when they should have expected three. Maybe they didn't develop the ability to distinguish between more and less until they were a year old. Maybe they hadn't developed the ability to hold the image of three in their mind long enough to feel surprised by the discovery of five.
By the time the child gets to school, addition and subtraction don't make sense, or the numbers don't “stick” in their brain long enough to be solvable. Lengths and other measurements don't seem to correlate with quantities, and the reason 505 and 550 are not equal is not particularly apparent.
Dyscalculia (or mathematics) can feel like an insurmountable enemy to a child, who cannot make sense of math class day after day.