LTNman
Yes, that's the difference between AMs and ATMs:
AMs (Aircraft Movements) - Any aircraft take-off or landing at an airport. These could be either commercial or non-commercial flights. For airport traffic purposes one arrival and one departure are counted as two movements.
ATMs (Air Transport Movements) - Landings or take-offs of aircraft engaged on the transport of passengers, freight or mail on commercial terms. All scheduled movements, including those operated empty, loaded charter and air taxi movements are included.
Source: CAA
If you look at the CAA 'Aircraft Movements' 2014 Annual data, you'll see there were just over 15,257 movements for business aviation, which comes under non-commercial (ie not included in ATMs).
That works out as 15% of the 101,950 AMs in 2014 (CAA figures). I'm no expert on LTNs traffic mix but I don't think there could be many more than that to account for ATMs. I'm not sure that any business aviation counts towards ATMs, though I could be wrong?
Nonetheless, the 20.4% in December was for ATMs only and I think it's fair to assume that's pretty much wholly down to the growth in mainstream passenger traffic.
For many airports in the current climate, you only have too look at the data to see that passenger growth is normally substantially higher than growth in ATMs. Below is the latest full data available (October 2015):
October 2015 - % Change on Oct 14
Airport / Terminal & Transit Pax / Passenger ATMs
LTN +22% +19%
LGW +8% +4%
MAN +4% 0%
STN +8% +5%
EDI +10% +2%
BHX +5% -3%