Deltav isn't calculated correctly for crafts with multiple engines when the LF or OX doesnt match each other.
Multiple Engines that use both LF+OX but have an uneven amount of each to draw from causes the deltav to be calculated incorrectly.
Two Simple Ways to make this happen:
1. The craft and screenshot show this with one fuel tank.
2. A player might have a LF tank and a LF+OX tank joined together for using both a standard engine + a nuclear engine
Note 1: If the Craft file has both the LF and OX equaling each other the deltav is correct.
Note 2: I have been using a Calculator and KER to confirm my suspicions, but do note that I have tested it without KER as well to make sure its not interfering
Somehow keep the LF and LF+OX tanks separated from each other somehow so the the standard engines aren't drawing from an uneven ratio