Sometimes it's the actual change in timing when coming to a stop while the throttle is closed, but the RPM's are higher than at a standstill,like when you come to a stop sign and push in the clutch and the RPM's drop for you and then they come back up, it's going through timing cells at different MAP and RPM changes, then it settles down when the motor catches up to itself. If you have no vacuum leaks and everything is working right, it should idle most if not all of the time in one fuel cell, usually the 20hg at 1000 RPM cell, unless you turn on the lights or the A/C, then it should go into a higher load cell and might bounce back and forth between different cells. Mine was starting out high when I stopped, like 1400 RPM, then settles in at about 1000 RPM when the motor is well warmed up, so I watched the cells being used at those points in matrix mode and adjusted fuel up or down to even things out. But I have much "bigger" cams than stock, which causes more overlap and needs more fuel and timing at idle to offset intake dilution from the extra overlap.
Also check your timing map between the idle and 1500,2000 rpm settings and make sure there's not a huge difference so it settles down correctly when coming to a stop. Yes, any addition to the timing upwards on the timing screens relative to "0" which is TDC ignition,is advance before top dead center. So if you adjust timing from 10-15* you advanced timing by 5*, if you adjust timing from 15*-10* you are retarding timing by 5*.Generally the higher the RPM's the more advance you need to a point, because the fuel burn rate is static at a given AFR,while the piston speed changes with RPM and will outrun the burn. Optimal timing for peak torque is usually about 15* ATDC, although you can only find that point exactly with an special sensor which is very expensive, or you have to try and find peak torque on a dyno. Again, the faster the motor spins the earlier it has to start combustion to reach peak torque at 15* ATDC.
