As far as I know...
The AI used to only build ships (and other stuff) if its cashflow was positive. This is a tad silly in this game, as income can be much higher than cashflow (bonus income being the source), and the AI would become filthy rich.
Some patches ago, a routine was added to make the AI take cash on hand into consideration, and spend some of it if there is lots of the kind. However, the build order screen is always recomputed when you open it, and this extra routine to take cash on hand into account is obviously only used when the computer does the AI check for what to build (once a turn or something).
This means that when it checks if it wants to build, it has spare cash (from the cash on hand), but when you open the build order screen linked, it looks only on cashflow and finds no available cash.
This is a feature that could possibly be promoted to a bug...