Wind is an intermittent source of energy.
Seasonally, it is lease productive in the warmest months, which are the months with the highest peak demand in Ontario.
The calculator uses a capacity value for wind generation (Capability at Peak) as implied by IESO 18-month forecast reports going back a number of years.
However, while it is true all sources sometimes fail to provide the expected capability at peak, wind is really only likely to hit the stated capability ~60% of the peak demand hours in the summer and about half of that for peak winter hours.
The calculator is, in my opinion, being overly generous to wind in using the IESO's figures - as they mean that, particularly in winter, wind would not be supplying much electiricity in ~20% of the coldest hours.