We will probably never know, but Charles Conlon was a photographer and like most artists I bet he was a bit of an eccentric personality. Today we hold him in such high esteem but in 1910 he was a just a guy with a camera who worked at a newspaper with a job like everyone else. You could have walked right into the "Evening Telegram" office 100 years ago and probably guys in his own company didn't know who he was. He lent his prints out, sent them to friends, sold them, gave them away as gifts and sent them to Baseball Magazine etc. Throughout his career he worked at all sorts of papers, worked for himself, and sent things all over the country. My guess is that if he was on the road and taking pictures sometimes he might develop some prints that day, other times at his studio, other times at the newspaper office etc. If he was sending a packet of photos to Baseball magazine maybe his job required a stamp whereas if he developed them at his home he just hand-wrote notes on the back etc. We really don't know that much about his life because he was not a big deal in his own time and died so long ago. I am sure it has something to do with whatever his job required at the time he took the photos, and if he was at a stage where he was working for himself he probably didn't put much weight into it.
Short answer, we don't know
NOTE: Other photographers were the same way. Louis Van Oeyen switched employers a lot and during the early 1920's almost all his work was uncredited as he was just one of about 20 staff photographers at NEA. His stamps and notes are all over the place as well. Paul Thompson was a war-Photographer over seas during WWI and others had similar stories.
Rhys