I setup my lineup under the Base Siege tab. I do not change it for different events. However, there is a huge difference in power between Lords league and other events. 
Lords League and resource raid listed power: 8717
Lordlympia and guild clash: 7055

So far support has told me to unequip and reequip my weapons, which I did. It has made no difference. I waited for the update to see if that fixed it... Nope. As a matter of fact, the difference started at around 700 power, and has grown to almost 1700 difference.

Support emailed me last night and told me it was because my line-ups are different..... Really? They are created under the same tab and I do not change them... How the hell are they different?

I vaguely recall a post a while back mentioning that power on reserve heroes in the bastion is only displayed during certain game modes. I don't remember details but maybe try pulling equipment on reserves and see if that equalizes the power

Lord league and resource raid shows your offense power with bastion included.
Guild clash shows just your main heroes defense power, even when you check opponent's base.
Training grounds show total defense power including bastion.