Export contact particle ID with EDEMpy

Hi anyone,

I am trying to get ids of particles in contact with each other using EDEMpy. I only want information of particle conatacting with particle and I have used two methods.
1.contactids = deck.timestep[tstep].contact.surfSurf.getIds()
2. contacts = deck.timestep[tstep].contact.surfSurf.getContacts()

contactids = contacts[:,[1,3]]

I previously thought these two methods should return identical results, both returns contact particle pairs.
However, I noticed that the two methods generate different results and I am confused about the reasons. I would like to know what exactly does the two functions: contact.surfSurf.getIds() and contact.surfSurf.getContacts() returns? Why is there a difference?

Many thanks,

Jason