Not a reach. She just was too injured and distracted by the fearsomeness that is Merle to put what she knew and what she just heard together
Making up excuses for this ho and sh*t
Ur just a.ssuming Andrea told exact details about the crew she use to roll with to Michonne. I mean I know it's a zombie apocalypse and the worlds getting smaller but i'm pretty sure she wouldn't put the pieces of the puzzle together since we don't know exactly Andrea and Michonne's conversations for the 8 months they were together. Maybe if she took a picture of the crew and gave it to Michonne and she still didn't bother to include the fact "oh yeah I know you guys from the picture" then I would be like, OK writers fu*ked up here. And hey maybe Andrea did tell her exact details about everybody in the crew and Michonne almost put the pieces together till she saw no pregnant woman or a bald head black dude name T-Dog so she must have thought to herself "well that doesn't match the group Andrea was with"
The last time Andrea was with the group sh*t was awhole lot different and the group was bigger. Hell she doesn't even realize Shane is dead. If you remember everybody in the group found out he was dead when they finally found a place of refuge miles away from the farm way after Andrea was split from them. Did Andrea even know Lori was pregnant? I can't remember if she knew from the 2nd season.
And AND when Andrea was with the group in the 2nd season Maggie,Herschel, and Beth wasn't even apart of the group officially. It was Rick's group and the Greene family but they didn't all group up till they were force too due to the zombie heard that took over the farm. Which again Andrea wasn't aware of
I mean it's so many ways you can go with this. BOTTOM LINE you reaching man