Maybe they're just not that smart....?
Perhaps they are behind where you were in terms of rote numeracy, but perhaps they have a deeper understanding of numerical objects than you did at that age?
I've spent pretty much my entire engineering career (25 years and counting) doing digital signal processing for realtime systems (voice coders, radio modulation and demodulation, GPS, inertial navigation, and graphics tomfoolery) and over time I've developed a pretty good grasp on numerical objects, algebra, and calculus, in fixed point, floating point, and modular field arithmetic. Certainly I know that stuff a lot better now than when I graduated, and I can think back through my schooling and see what was and what wasn't effective, from the basics through to a decently high level of applied math.
What I see my kids being taught, is basically a shotgun approach; but they spend so much time blasting them with alternate methods for doing things, that there is no time to teach the kids the underlying fundamentals which might help them tie things together; and the kids get confused between the different parts of the different methods so that instead of learning one or two methods fully and practicing it until they have it cold, they learn five methods superficially and forget the solution processes two days after the math unit ends.
FWIW, I did my grade school curriculum in the Netherlands in the 70's and it was like this (from a math perspective): Grade1: Addition/subtraction; Grade2: Multiplication tables. Lots of recitation to drive the numbers into your head. Grade 3: Long division. Grade 4: Fractions. Grade 5: Decimals and bigger numbers. Grade 6: Common factor elimination in fractional expressions.
My kids are three to four years behind that timeline because of the unnecessary fluffery that seems to pervade North American education.