Epub is epub and devices are devices. With iPad ignoring text-align: center; there is a lot of extra work going on out there to put in substitute elements to make ePub work with devices. Every new device presents new "challenges".
The most common hack to overcome this iPad problem is to introduce a blank <span> statement into the paragraph. It can be a lot of work and looks like this:
<p class="center-me"><span>I am text that wants to be centered</span></p>