I strongly believe this has less to do with HOW math is being taught and WHO the math is being taught to. Kids cannot be failed in earlier grades*, so more students who have not met the earlier requirements are making it through to more advanced grades without having the skills to complete the tests. In the past, you would have had children of different ages within a couple years taking the tests because they would have been held back. Some of the students in grade 6 would be 12-13 others 10-11. Now they're all the same age but at vastly different skill levels because they're allowed to advanced without picking up the requisite knowledge first.
*There are vast developmental differences in single year increments at those ages and holding students backs actually broadens the developmental issues later on down the line, setting them on a worse trajectory than if they just let them through. While I get that people resist this idea because we're not allowing kids to fail, it's actually better in the long term. The evidence strongly shows that holding young students back actually creates far worse problems later in life, especially when they are teenagers.