According to the field comment, CAddrInfo::nRefCount represents "
reference count in new sets (memory only)".
But since the commit e6b343d880 [1], each address deterministically hashes to a single fixed location in the "new" and "tried" tables.
So the "new" table will always have only one entry with the same address.
Before this commit, the same address could be inserted in multiple entries (at least, 8 entries as defined in ADDRMAN_NEW_BUCKETS_PER_ADDRESS), so it made sense to keep track how many reference to same addresses were in the "new" table.
If the purpose is to know if the address is in "new" or "tried" table, wouldn't a boolean field do the trick ?
[1]
https://github.com/bitcoin/bitcoin/commit/e6b343d880f50d52390c5af8623afa15fcbc65a2