Unable to add a ship to get to the fleet cap and create a fleet

some of the ships you get in the campaign have modules that reduce the logistics cost by 50%. i.e. the medium ships with that mod cost 2.5 instead of 5, the large ships cost 3.5 instead of 7 and the huge ship costs 4.5 instead of 9 (i think - not sure if the base cost for huge is 9 or 10). if you add up the numbers manually, the fleet probably has 25.5 points so you can't add +1 without getting over the cap of 26.

so it's more of a display issue, but it only affects the campaign since that special module doesn't exist outside of the campaign missions.

If you hover over the logistics of any particular fleet's info screen you can see the unrounded number and what is contributing to it in the fleet's logistics tooltip.