Usually for bridge designs they are up to the consultant that the MTO hires to design the bridge for them. Local suppliers for pre-cast concrete and steel can vary a bit by location, so that can have an impact on bridge design (ie. whether concrete or steel) as well.
As for Northumberland Road, that bridge crosses the 401 at enough of a skew that they decided to go with a cast in place, post-tensioned bridge design. During the late 60s through to the early 90s most bridges in southcentral Ontario were built this way. Post-tensioned bridges are a bit more of a pain to construct because you have to build temporary formwork over the highway while you pour the concrete in place, but offer the designer more flexibility in terms of span length. In this case, the span length is a bit longer than a standard bridge because Northumberland crosses the 401 on a skew.
edited to add:
it is funny how much narrower the bridge over the 401 is for 59 relative to to Highway 19 or Wellington Road for example.
Looking at streetview, the slope paving that protects the abutment is almost right up to the edge of the 401 at the 59 bridge. At Highway 19, for example, the slope paving starts something like 28 metres from the central highway median. It's wide enough that they could widen the 401 to ten lanes and add loop ramps to the Highway 19 interchange converting it to a conventional parclo without touching the slope paving. When they get to widening the 401 through Woodstock, they'll pretty much have to remove all of the newly built slope paving and construct a new abutment ballast wall. Definitely a different design philosophy for those two bridges.
edited again: I highlighted what I mean by slope paving. I'm guessing not everyone reading this would know what I mean by that: