SA fly line weight question

Are these line weights marked on the boxes the same as buying true to weight lines of other sizes? Like the Sonar titan line pictured. Would that be a normal size 11wt line if bought this in size 9wt? And if I wanted a “true to weight” would I need to buy a 7wt line for my 9wt rod?

Also, I own the amplitude smooth fly line in “true to weight” which is what I’d want for my 4 weight which pretty much only sees dry flies. I’m looking at the same line or textured version for a 5wt but this rod would be more for nymphing or dries on bigger water so I thought about over-lining it and buying a 6wt line but wound this “half size heavy” option be better? Is the 6wt line the same as a “one size heavy” 5wt line?

If so, what’s the purpose of all these options for line weights? The half sizes I can see but one or 2 sizes heavy why not just sell it as the true to weight line of the appropriate size and let anglers choose their size ups themselves?

I live in a more remote area and have to buy online so can’t go try all these at a shop somewhere. Just want to make the best informed decision before I purchase.

Thanks