Whats wrong with your calculations: you forget about attack power. It affects abilities that deals % of weapon damage too. Formula:
Many abilities uses a normalized damage. Source: http://wow.gamepedia.com/Normalization
For example, if we have 13110 - 19666 weapon damage and 30000 attack power:
Minimum damage will be: 13110 + (3.3 * 30000 / 14) = 20181
Maximum damage will be: 19666 + (3.3 * 30000 / 14) = 26737
And then we can add slam % damage + base damage, all modifiers like seasoned soldier, pvp power, resilience and etc.
The same with http://forum.pandawow.ru/showthread.php?t=183367