My crab, Pinky, is well known to be a notorious shell-swapper, but I started to notice a trend
Sunday-Monday shell:
Tuesday shell:
Wednesday seems to be random shell day following no pattern.
Thursday shell:
Friday-Saturday shell:
This is so weird, I did a test, come tuesday I took out her beloved polka-dot shell and waited until her usual time to come out (another pattern, always about an hour after lights out) And when she came out she went straight for the shell shop. I was amazed at this behaviour and even more amazed at her reaction when she couldn't find it. She started throwing shells out of the shell shop! Poor Cheeto was on the bottom level and almost got conked by a falling shell. She seemed really upset, so I put the shell in front of her and she hopped right into it. I tried to do this test again, but she only ran around frantically until I put the shell in the second and third time. Do you think she could have some sort of disorder? Or that her brains out of whack?
I forgot to mention, she also does everything at the same time. Her routine is: Come up an hour after lights out, run up to the shell shop, switch (or just play around if its still saturday) down to the moss pit, she plays (or eats) some moss, down to the bottom level, eats some dinner (by this time Draco is usually up and hot on her tail) Then a quick dip in the saltwater and a swim in the freshwater. Or sometimes she just sits in the freshwater for awhile. Then she likes to crawl on the vines and plants and run around in the running area. Then shes usually back underground 4 hours after coming up, yes, I stayed up all night and observed her a few times 
she was running around in circles last night and he just sat and watched her, and then started following her! They're so darn cute, I'll be heartbroken if she has eggs 
because if SojMad and Wolfnipplechips can't get the babies to live with years of experience under their belt, I doubt I'll be able to 
Have you ever gone shopping in the sale racks and found three pairs of perfect shoes but could only afford to take one? How long does it take to make that choice and how many times will you second guess yourself and go back? And you know that if you were to come back later there's a good chance those other two pairs won't be there. 
Crabs are naturally creatures of habit - they come out the same time every night, and they have the same urges from day to day. It's a bit odd that she likes the same shells, but I don't think it's too weird. Those are the ones that fit the best and none are better or worse than the others, so she can't decide which is better. And since they aren't being snatched up by another crab she's having days and weeks to languish over the decision. 
