I was wondering even if the aircraft seem to be 50 to 100 miles or more south of Halifax they still talk to Moncton center?
Yes, the Moncton Sector extends to about 160 miles south of Halifax.
You can pull up the High Altitude chart here (click World Hi) ...https://skyvector.com/
Unfortunately the CTA sector boundary between Moncton (VHF) and New York Oceanic (HF) is not completely depicted on the chart. If you contiune the line shown through AVAST the CTA border extends to NOVOK then to JEBBY, anything north of that is talking to Moncton on VHF.
This is not to be confused with the FIR boundary that is depicted on the chart, this only applies from FL280 and below. Between the FIR and CTA boundaries Moncton (VHF) owns the airspace from FL290 and above while New York (HF) owns from FL280 and below. Probably because of reception range issues at the lower altitudes from the Yarmouth and Halifax RCAG's ?