Yes, silver is notorious for tarnishing. Great when new, no so great when left for a long period.
Silver tarnishes within days exposed to atmosphere, they need to be delivered in vacuum sealed packs.
Spraying them with the rosin based flux you can get in aerosols will protect them but the boards stay tacky and are more likely to need washing after assembly.
Edit after watching the video:
You generally don't want silver plating because of the tarnishing problem. A pad with solder finish is always going to solder easier than one with silver. The problem with hot air solder leveled (HASL) is poor control of how much solder is left on the pads and that really is a problem for fine pitch SMT parts where solder on the pad reduces the amount of paste (and importantly flux) that gets stenciled on to it, it also prevents parts sitting flat on the PCB. Gold of course is the expensive answer the other down side being risk of embittlement so some people think silver is the safer option for difficult parts like QFNs.
On the silk screening the difference looks to be the NZ ones were not screened but photo imaged - cost again. Same might apply to solder resist differences.
On the high cost of panels - they don't fit into the prototype flows which give the cheap prices. When you buy 50 boards on a prototype flow they put one of your boards on a big panel along with one of everyone elses boards and make 50 of those big panels. You share the set up costs with everyone else on the big panel and get the volume savings making 50 of the same. When you buy 50 boards in panels of 10 it is harder to fit your larger panel in with other boards and they would only need yours on 5 big panels not 50. The cheap prototype flows are all about using all the space on a large panel and making as many of the same large panel as possible. That is why you get big discounts for longer turn around, it gives them more opportunity to fit your boards in with others.
It is difficult to calculate true costs for these flows, I have had automated quotes like it was cheaper to buy 10 boards and throw two away than it was to buy 8.