Enabling crossfeed thru SRBs sometimes reduces displayed burn time, dV?

[Anaxagoras]

Wondering if this is a bug others are seeing or if it's just me:

Around half of the time or more, enabling crossfeed on SRB's decouplers in the VAB reduces the SRB's displayed burn times and dV.

For instance, a Thumper's displayed burn time drops from 42s to 33s.

This doesn't appear to change the way the rocket actually behaves in flight; the boosters still burn for their appointed time. But it makes a mess of trying to design the rocket accurately.

(It took me a good while to realize this was even possible, that you could attach liquid fuel tanks to your SRBs and enable crossfeed and the fuel would actually flow through the SRB, giving a half-asparagus effect. Drop tanks without the extra drag and without the cost of extra decouplers, even without having to run any fuel lines.  Works well if you can time it so the tanks are empty right about when the SRBs flame out. This displayed burn time issue makes it a little less convenient to figure how much fuel to put there, as well as throwing off the overall dV calculation.)