Blizzard stated awhile back that we have to look at it like this:
1-60 - We are fresh adventurers dealing with the current threat in current times, arthas is dead, illidan is dead. Deathwing/Cho'gall are destroying the land!
60-70 - (whether you like it or not) We go back in time to Outlands dealing with the Burning Legion invasion, illidan is alive and well.
70-80 - The Burning Legion has been dealt with, Kaeljaedan has been stumped. ZOIKS! SCOURGE INVASION IN NORTHREND! Arthas is chillin (quite literally) on his throne
80-85 - Arthas has been defeated ZOMG DEATHWING is ruining the world!!!!
TL
R - you are always in the time of the expansion your in