Because the maximum coin win, per line, is 500 coins. This, of course, is then multiplied by your own bet. Because this changes by person, it's better to list it as 500 coins, rather than the biggest win possible (as this might be misleading).
Some people will only bet 1 coin (actually 30) = 500, whereas others will play 10 coins (300) = 5000.
That's only on a coin value of 1p/1c - you can change the coin value to 2p/2c or 5p/5c etc meaning the top win obviously gets bigger and bigger....
1p/1c = 500 coins at that value (£5.00, then multiply by bet)
2p/2c = 500 coins at that value (£10.00, then multiply by bet)
5p/5c = 500 coins at that value (£25.00, then multiply by bet)
Then what about the slot gam Robin Hood Feather? It pays out over 1 million coins, and has 243 ways to win, does the same calculation as the alaska slot count here too?