Alfakhro

Silver bars are like some other products, for whatever reason people get a liking for a particular make and these always appear to have a higher premium than others. This is particularly so for older bars. JM bars are an example of this🤔

I can think of a few possible reasons:
Private Mint vs. Government Mint - Government Mint products might carry a slightly higher premium because of a favored legal status (nominal legal tender status on coins) or just trust / name recognition.  They may be easier to trade in times of market illiquidity - especially to folks who are n00bs to metals markets. Production Cost - Some bars include security features like serial numbers, laser micro engraving, radial lines, holographic images, etc.  The fancier the features, the more expensive the production costs and the higher the initial premium from the Mint. Purity - Some might be .999 (3 nines fine) and some might be .9999 (4 nines).  The extra purity requires a more stringent refining process. Art - Some bars might just look nicer, be stackable or otherwise have greater appeal to the market