When did you change your plan? If you changed your plan in middle of the last billing cycle, only a partial of the 75Gb was available and transfered to the next month as rollover data. 

https://www.koodomobile.com/en/help/proration

The 50GB->75GB upgrade was fairly recent so I’m not expecting 75GB as the rollover, however I am expecting closer to 50GB as the rollover than the 12.5GB that was recorded since I used less than 0.06GB.

I think you didn't check the link above. If you switch in middle of a billing cycle. You only have like 75/30 x few days of data in the data bucket. Your 50Gb was lost once you switched plan.

Ah. Silly me, I expected to carry a prorated portion of my data balance over when I upgraded my package. Having my data balance zeroed and starting with a prorated portion of the new plan’s data isn’t logical, so I never considered that. Thanks for clarifying.

For the record, had Koodo been a classmate when I was in school, they would not have fared well in math class.

Proration, properly done, should apply to both the 50GB and 75GB. As it is, the lesson to take away from this is “don’t change phone plans late in your billing cycle”. Had I used, say, 45GB of the (80ish% of 50GB) + (20ish% of 75GB) = 55ishGB I paid for, I would have ended up with a 30ishGB data overage charge.

The pro-ration is to prevent abuse.

So if a customer with a 50GB plan uses all of their data 25% of the way through their billing cycle and then switches to a 100Gb plan for the remaining 75% of their billing system they should get 12.5gb for that 25% period and 75gb for that remaining 75% period.  This is because the customer only paid 25% of the 50GB plan and 75% of the 100GB plan.

A plan change for more data is not supposed to be used to deal with data overage.  It is purely a plan for long term use.

At least there is method to the madness, then. Thanks for clarifying, Dennis.