I'm fairly certain that there's a bug in PDC production when more than 1 PDC of a particular type is produced in a 5-day.
I designed a fairly cheap fighter base and launched a build of 8 on Earth. A while later I checked, and noticed there was 1 of them in my PDC task group, and the count remaining was 5.8-ish. I thought "wait, where's the 2nd one?" then decided I'd probably just had a little too much Bailey's when I launched the task. I bumped the number back up to 6.8 and didn't think about it more. A while passed and the same thing happened - (built)+(sheduled)<8. I bumped it up again, and this time watched more carefully. I started out at ~5.6, now there's 2.66 left on the task, but only 1 appeared. I assume that what's going on is that the PDC code is assuming that only 1 PDC needs to be added to the TG, even if more than 1 was completed in that 5-day, and the extras all "fell off the truck".
Now that I know what's going on, I'll just insta-oob the phantoms back into existence.
John