Yes, you are correct and currently it's WAI.
The rationale is to avoid extra seats to be assigned as I had instances where of the 311 seats you may ended up with 314. In all my tests the ROUNDDOWN always created less discrepancies than the ROUNDUP with results similar to yours usually happens only when a high spoil amount of votes are present.
In your case, If you scroll down to the Majority of the House section in the Government Federation calculation, you will see that the Majority has a ROUNDDOWN formula, effectively correcting the "missing" seats. Other governments have instead a ROUNDUP majority, based on the rest of the formulas.
You can change to a ROUNDUP formula for your own sheet, and if so you may end up with elected seats being higher than the maximum amount of seats, which is not a biggie. However, I would recommend though that you also change the Majority of the House formula to the ROUNDUP version and amend for the extra not accounted seats.
As all calculations happen at the Government Federation sheet level, the Majority number in the ELECT page can be considered only a "Visual" bug and since that entry is manual, you can enter the effective right number without compromising anything.
EDIT: Please remember to copy only GREEN elections in there, as I have a feeling you copied a RED one in the file.