Depending on tarrif/company. Your two prices, are no split down the middle for each month. Some companies will make you use expensive units in winter, drastically increasing what you pay and fooling comparison sites as they don't take it into account.
So where is says 687kw at Xp then yp after that.
Some go 3/4 of that allowance in winter. When you'll actually use fuel.
But does that actually matter? as the Xp is the high amount and Yp cheaper, so you will use that 687Kw no matter how it is split - I use 4600Kw / year and am not that high a user.