My theory is that the flashbacks we saw in earlier seasons were actually part of the ALT universe.
The ALT universe started when they traveled to the 1970's.
Just arriving changed the future.
Locke appeared and had discussions with Richard. Richard thus went to the real world off-island for Locke's birth and to test him. I'll get right back to this after this paragraph.
Remember how in the ALT the Losties seem to get a recognition toward events on the island? I.e. Kate looking at Jack through cab, Jack's appendix scar, etc.
Back to Locke and Richard. Remember when Richard saw Locke drawing the Smoke monster? This is part of that remembering that we see in the ALT. This suggests that some flashbacks were actually already ALT's. Though the same events may have happened, we saw some ALT twists added. And other events may not have happened in the original time line but we were lead to believe that they were.
I'm sorry that I'm not exactly sure how to properly word it but you get the point.
Thanks for reading ☺☻☺ (those are smiley faces ☺☻☺)