I think they want to keep it looking like the momentum is building. There isn't any reason why 350k+ couldn't be done this week but next week supply won't be as large so they'd have to cut back. See underlined.
51k done this week so far.
26k J&J in the fridge, all of which could be used now.
There's about 358k of Pfizer and 106k of Moderna sitting in the fridge after Tuesday. C. 300k second doses of those two due over the remainder of this month I think. So they could do about 164k additional 1st doses this week of those two and still have enough reserve amounts to cover those second doses (some of which will be used over the remainder of this week - maybe 50k as a guesstimate, haven't checked but would make sense). Including the first and second doses still to go, that's 214k of those two that could be used between Wed to Sunday.
There's also still a lot of slack with AZ. There's about 150k in the fridge and they only have to do 56k second doses this month. That's 94k further that could be done with AZ this week and still hold back enough for the second doses.
That's 385k doses that could be done this week, and still have enough held back for the remainder of the second doses due this month, with weekly deliveries due. Thing is though that the 350k likely couldn't be replicated next week as the AZ, J&J and Pfizer buffer would be used up with deliveries of just 175k Pfizer, 33k Moderna and 31k AZ due next week (75k J&J are due over the next two weeks). You'd realistically be back down to 250k or so, which would be drop that they wouldn't want to explain to the media.
I'd say they will just continue to look to ramp it up, under promise and over deliver. 240k+ this week (220-240k primised). Probably 270k+ (250-270k estimated) next week. Then maybe a 300k week and then a 350k week to finish off May. Looks nicer on a graph and they also can keep the mobilisation ramp up - maybe pharmacists by the end of the month?