What a marvellous question! In three years on type I never once looked at this table, just accepted that it happened and if I did not like the result I intervened...
Try this for my understanding of the AB logic in that table.....
In the climb, passing FL 245, if your SELECTED speed is 320,335, or 350, it will switch to the appropriate MACH.
If not it looks again at FL 255, and if the SEL SPD is 310,325, or 330 (or above I guess) it switches to MACH.
And so on.
In descent it will switch at the first spd/alt crossover, depending this time on MACH. So at M.78 it switches at FL 285 to 300 kt. If your descent started at FL 280 it would wait until FL 265/315 kt.
That is my interpretation - it could be wrong! On a cautionary note it is described as SPEED/MACH SWITCHING and NOT vice versa. The written words only describe SPD to MACH. The general precision of AB documents lead me to suspect it does not apply on descent - but again I could be wrong...