You've not forced your line to run faster at a lower dB, its a quirk of the system in your case, your modem came up quicker than others and therefore the system see's a low level of noise and sets the line accordingly. The other modems then come online creating additional noise and your dB reading drops, at the time of sync it would have been 6dB. If on a Huawei cabinet then these use dBx and are designed to sync at lower than 6dB.
In some cases lines have known to stay synced at even 1dB, but if they resync they will go back to 6dB target unless on dBx.