It's because the max attainable value can be calculated using different parameters than what the line will actually have to use.

The "basic" max attainable rate method in G.993.2 left various aspects undefined, and I expect most manufacturers filled in the blanks to give the highest possible value.

Essentially, the max attainable value is calculated using interleaving parameters (such as depth, block size, amount of FEC) chosen to give maximum bandwidth. It has to stay within the max delay parameter, but it does not need to meet the min INP parameter.

For the actual speed, the interleaving parameters will have to be chosen to meet the min INP level specified.