Hi, I have the same issue using woocommerce 2.4.7. I have entered the prices without tax and enabled the tax calculation. On a single product it works fine. On a variable product it shows the price range without tax. Once a variable is chosen it adds the tax.
I tried all, even deleted the whole wordpress installation and started from scratch. No luck.
I installed woocommerce 2.4.6. again and the tax issue is gone.
So it seems it is a bug in the woocommerce 2.4.7 version. At least for me.