Assuming it was running while you pulledbthe dip stick.... this would be caused by the piston's motion. When running the bottom side if the engine works like an air pump so to speak, this is why the crank case is vented, to allow this air preasure to escape. If it is an over excessive amount of air you could have a little blow by and eventually a rebuild (new rings) will be in order. But some should be expected. I wouldn't recommend pulling the dip stick to check or add oil when running.
2012 SPORTSMAN 800 EFI (INDY RED) Sept. 2015 ATV Of The Month
Mods: Polaris 2500HD Winch - MotoAlliance Line Stopper - Glacier3 60" Plow - Polaris Rack Extenders - Polaris Bumpers - LowPro Shield - Polaris Hand Guards - Warning Debadge - 3Way Head Light (Relay) - Cabelas Mirrors - Wes Ind. Std Seat/Box - Kolpin Fuel Pack Jr - 26" Kenda BearClaw HTR Tires - HighLifter HD Springs - MotoAlliance Sirius 6" Pro LED Light Bar
2005 SPORTSMAN 400 (SONIC BLUE)
Mods: Warn 2500 Winch - MotoAlliance Line Stopper - Polaris Mid Shield - Wes Ind. Seat/Box - Shift Interlock Delete - Coleman Mirrors - Polaris Bumpers - 25" Kenda K590 Tires - PVC Spring Spacers
2014 RZR 570 EFI EPS LE TRAIL (WHITE LIGHTENING)
Mods: HarborFreight Badland 3500 Winch - MotoAlliance Winch Mount - MotoAlliance Line Stopper - Polaris Sport Roof - 1/2 Windshield - Warning Debadge - Rear View Mirror - Seat Adjuster - 26" Maxxis Tires - MotoAlliance Sirius 10" Pro LED Light Bar