Slide to answer vs...

So I've been using an iPhone for work since the iPhone 4, and currently have a 5s. My personal phone currently WP 8.1, which replaced my up to date HTC One. You would think moving between all these devices on a regular basis would be a little jarring, but it's really not that bad.

But one thing within iOS is jarring without fail: Sometimes to answer the phone I must tap a button, other times I have to slide. It is confusing and I've accidentally hangup more times than I can count. Is this a bug, or by design? And if by design, what rule is governing which answering mechanism will show?