Your figures most likely are correct. But what you have to look at also is the runs made by their response neighbors. In this case, Engine 45, 46, & 48. If you have the time, pick a five year period. See who was in service in the area in those fire houses during those years. It might be hard to find out what the response policy was back then, but it would interesting to see what you find.
Also check Eng 42, 79, & 90. I know that 88 never ran with 75 on the box but they did with the three previous mentioned units. But remember that Sq. 1 & TAC 512 were also in their area during some of those years. Then you just might find out Where's The Fire? Eng 88's work has aways been to the south of their quarters. So also between them & 48, but mostly toward 45 & 46 quarters.
Yes, on some boxes both units went, on others only one unit responded. This plan help cut down on the running of some nearby companies. Also take into consideration that response levels were different. Three engines on the box & also the second alarm, not four like today. If 88 was third due on the box, the second section did not respond on the second alarm unless special called. this would keep a company in the area & reduce needed relocations. In some cases, relocations were started on receipt of the old radio signal 10-30. Rescue 3 was on the third alarm in some battalions back then if I remember correctly.
Something else to take into consideration is FDNY's police about giving credit for runs to relocating units. For a good number of years, the credit for a run & also a worker went to the unit who quarters a company responded from. Example Engine 88 was relocated to City Island & had two runs during their stay there. The credit years ago went to Eng 70 & not 88. I think that has since changed to where now 88 would get the credit.