I agree. They r here, no matter which month. 7th month was set aside by our forefathers to remember the dead, to give offerings. The tradition has been carried on till now.
As to why there seems to be more ghostly occurrences during the 7th month, imo,
We r more conscious abt ghosts during the 7th month. Hence, anything that seems out of the ordinary, we call it ghostly encounters.