DTI max out before reaching maximum number of mortgages?

[Spencer Funk]

Hi everyone. I have been studying the craft (reading books and listening to podcasts) and have heard it mentioned that some people will surpass the threshold for their DTI radio (~36%) long before reaching the maximum number of mortgages that major banks will issue (~10). My question is how? As long as folks are collecting more in rent than paying out then each property should actually HELP the DTI ratio, right?

I appreciate anyone helping me understand a bit better. I'm sure I'm missing something here. Thanks!