The 18Mbps figure is the theoretical maximum sync rate for your line based on the conditions of the line. The 16Mbps figure is what your modem is actually sync'd at. Your line is capped at 16Mbps I am presuming for stability reasons. The cap is set to 16000Kbps and the modem is sync'd at 15993Kbps with a Signal to Noise Ratio of 6.1dB which is good, we prefer to have a minimum of 6.0dB SNR.
We can raise your cap if you like, to see if you can sync at a better rate though it may come at the cost of stability. Adjusting cap rates is intrusive so you'd lose internet for a few minutes while we implement it.