When a bot (or anybody else with the nominornewtalk right) edits your talk page, no "You have new messages" alert will be shown, unless there are also edits by regular users. But when there are both edits by regular users and bots, the number of all edits and all users including the bots will be shown in this message.
E.g. when a user makes two edits to your talk page, followed by one minor bot edit, you will be informed that
You have 3 new messages by 2 users.
Instead, the bot edit should be ignored (as it would be if you had been online between the edits of the user and the bot), the alert should say:
You have 2 new messages by 1 user.