That's just it, costs are tiered, the top tier is $0.40/kWh, but the tiers before are significantly lower. Depending on your consumption it could average out to $0.20 (they charge you tier 1 for the first xxx kwh, then tier 2 for the next xxxx kwh, then finally tier 3 for every kwh after that).
If you average it out, you will get a deceptively low value. Yes, your average Kwh may cost you 23 cents. But if each additional Kwh costs you 40 cents, that's the number you should use when considering the profitability of mining.
That's the number you should use if you consider expansion of mining perhaps, but for profitability you should calculate using the numbers that are applicable to your draw.