There is no error on the contribution limit, so far as I know.
Actually, the 2018 HSA Family contribution limit is $6,900 for HSA owners under 55. The IRS at first announced in 2017 that it would be $6,900, then in early 2018 changed it to $6,850, then later in 2018, changed it back to $6,900, which is the final, and correct, number.
The annual HSA contribution limit can be reduced based on circumstances. For example, the annual contribution limit can be pro-rated by the number of months you are covered by HDHP insurance. However, this does not explain the $6,750, because 11 months of Family coverage is $6,142, far smaller than $6,750.
One way this could happen is if you and your spouse both had HSAs. Even though each spouse is allowed to have any HSA, the annual contribution limit of $6,900 must be shared by the two spouses when they have Family coverage. So one spouse could be limited to $6,750 if the other spouse needed $150 of the shared limit.
If you have paid for the return and can see the forms 8889, please look at the Line 6 Smart Worksheet, and see if line c on one of those worksheets is $6,750. Note that this amount is carried to the other spouse's 8889 as the limit.
I suppose that you might have mixed going back and forth between Family and Self coverage to get $6,750 as the limit, but this is improbable.
If the discussion above does not address your situation, please post a new question here on AnswerXchange giving the details: one HSA or two, the type of HDHP coverage for each month, the age of each HSA owner, the amount of contributions to each HSA, any excess funding from 2017 carried over, and so on.
