Generally true. Some files are transferred to mississauga, seemingly files that are relatively simple and from countries with fewer security or other special considerations. Eg some India files go to mississauga, and mississauga seems to process more quickly (likely because the files themselves are 'easier' cases). Most Russia applications (if not all) go to Warsaw.
There have been some files that go to miss and then get transferred to field offices. Possibly the sort between easy/complex was mistaken. Perhaps there are staff with specific skills or knowledge or responsibility in one office and not another.
Note these are generalisations. They may transfer to specific offices for other reasons - workloads, capacity, backlogs, staff training, procedural pilots or other reasons we have no insight into.
And note, 'easy' or 'complex' is not a real term. It may be more like standard / non-standard or something similar. In other words, don't over-react to news it went to one office or another.
Well, here's hoping we are an 'easier' case and the PA paperwork goes to Mississauga, then.