It's probably by design (by Apple) due to the iPad having a larger screen than the iPhone and, therefore, being able to display more information without having to hide the address bar to give the page content as much space as possible.
Unfortunately, I do not have an iPad on which to test so this is just speculation on my part, although this seems to be the general consensus of opinion in this (old) Apple forum thread: https://discussions.apple.com/thread/5528272

Are your iPad and iPhone running the same version of iOS? (What version is it?)
Apple have changed the way Mobile Safari works (with regard to full screen content and hiding toolbars) several times over the years.
For example, they introduced the 'mimimal-ui' viewport property in iOS 7 and removed it from iOS 8. (Try a web search for 'Apple minimal-ui' for more information.)

Also, it might help to know if you are referring to a full-browser Juicebox gallery page (a 100% x 100% gallery with no other content) or a web page with an embedded Juicebox gallery and other content (including vertical scrollbars).
Try viewing both types of pages (and also a page not containing a Juicebox gallery) to see if what you are reporting happens with all page types.

I suspect it's just the way that Mobile Safari works on the iPad, though.