Maybe Alice is online more often than others. Or perhaps it just happened, it's normal to expect that sometimes a user might coinjoin with another more often, because the coordinator is supposedly selecting randomly. So, it's a matter of possibilities. Doesn't seem like the coinjoin is weak, though. Bob is simultaneously coinjoining with other people as well.

Why don't you ask in their Telegram? I'm not Samourai contributor. Here you go: https://t.me/SamouraiWallet.