beacuse the engines spool up when you select Conf1
Why? When you configure your normally slowing down. Let's say you're at 220 knots selected speed, descending in OP DES (idle thrust) and ATC request 180 knots. "Manage" the speed to start reducing to GN DOT (clean spd approx 205 knots) and take F1 (slats only which takes about 8-10 seconds), target speed now "S" speed (approx 185 knots). You're still descending in OP DES and thrust will only start to spool as you approach level.
Your speed is controlled by pitch (not thrust which will be at idle) in OP DES. In "managed" DES the profile is FIXED and speed is controlled by thrust so if above or on profile even DES should result in idle thrust.
Of course if you held 220 knots and select F1 then yes the engines would spool - but why would you do that? You're then putting power against drag.
I still think it's speedbrakes
but I remember being a pax in a BAe 146 many moons ago and the aerodynamic noise on flap retraction after departure was UNBELIEVABLY LOUD! So may be there's a bit of slat stuff as well. (I know the 146 didn't have slats - or did it?)