the attainable is the modem estimating what the line will achieve on its target noise margin which is 6db for the FTTC product.
IF your current snrm is below the target then the attainable will be reported as lower than the existing speed, the majority of the time this will mean if you resync then the speed will drop down to close to the attainable speed, instances where it may not drop is if bitswapping has disabled tones, but those tones will come back on a resync.