It's because of the different wireless adapters.
The iPad 4th generation has less bands while the iPhone 6 has more. This allows the iPhone to take advantage of more bands resulting in faster data speeds.
Only a certain amount of the extra speed is because of the processing speed boost between the two. The iPhone 6 and iPad 4th generation are pretty close in speed anyways. It's the wireless card, or adapter, causing the speed difference.
Hope this helped.
I may be a little wrong, but I'm almost positive I'm right.