Proffessional reprints indeed exist since worldwide market for these things is real and collectors tend to pay a lot. I do not know who makes them but many sellers, especially those on ebay who sell hundreds of old albums are obviously able to get them. They buy old albums, fill them with missing stickers and sell at premium price. There might be cutouts from printed versions too, as Rojocapo wrote, but those are rather amateurs, pros do get quality reprints.
Since nearly all of those albums are authentic, the only way to differ old stickers from new ones is if these are laser printed, while original ones were of course offset printed. You can google how to see the difference. If stickers are glued, and they normally are, this is more or less the only simple way to tell the difference. Other would be using proffessional services by some paper forensic company or paper age dating lab.
Of course there are enough authentic albums too, but you basically never know what you are buying. This is not only Panini related, simmilar stories are happening with old used tickets and ticket stubs andother printed stuff.