Why are they not the same, and why should they be different?

Because that's the law. I imagine the law is that way because people being openly allowed to indefinitely display offensive & provocative things in public areas wouldn't be conducive to the public good.

I reckon [I] could probably shout about as far as a small banner is visible with the naked eye

That would be a one-off act as opposed to a permanent display. If you permanently shouted "white silence is violence" you'd also be made to stop.

I assume you're supportive of those cases of police nicking people for saying off-taste stuff on Twitter, since that's a visual format like a banner, not a spoken format?

No because they are actually being criminally charged as opposed to being asked to take something down. Huge difference there.

This is not about the presence of a banner, but the contents of the banner:

I never said it wasn't.