Originally Posted by williaty
And no turbo, btw
Well THERE is your problem....
I have no specific knowledge of the factory Subaru ECU, but this type of strategy is typically done for one of two reasons:
1) Typically, some amount of fuel gets "stuck" on the walls of the intake ports with a port injected engine. Go into fuel shutoff (on a shift, long decel, etc.) and you tend to suck all that leftover fuel through the engine. When you turn fuel back on (after a shift) you need to throw "a little extra" in to make up for the wall etting effects. If this was "perfect" you wouldn't see it on the A/F meter, but it's safer to make things a bit rich here to cover production variability. (If you're a bit lean, the car stumbles when you get back on it)
2) More likely this is for emissions. When you're in fuel shutoff (see above) the catalyst(s) take all that extra oxygen you're pumping through the engine and store it up. When you turn fuel back on, the high temperatures and "extra" O2 stored in the cat make the car puke NOx. You can't make this NOx burp go completely away, but if you dump in some extra fuel, you can deplete the extra O2 quicker and reduce the effects on your cumulative tailpipe numbers. Incidentally, it also hurts fuel economy a teeny bit...
Your mileage may vary.