Looking for more candidates - is this normal?

Hi - I'm applying for my first big job and I'm nervous and excited waiting to hear back from a company I really wanna work for. They told me they were interviewing 7 people over 2 days (I was the first person they saw), and the interview went GREAT. Throughout the process he kept saying, "that's a great answer", nodding enthusiastically, at one point told me I'd set a high bar for the other candidates, etc - so either he was way too nice and police, or I did well. This was last Thursday. I was told I'd hear from them around this time (end of this week).

Yesterday I get an email from the company saying:

Dear Acrobatic

Thank you for taking the time to come in for the position, please note that we have some more candidates to see before we are able to communicate the outcome. We anticipate that we will be in a poistion to update you w/c 20th May. I am sorry for the slight delay and appreciate your patience while we conclude this process.

Basically I want to know if I should be worried? Because they're clearly now widening the search and interviewing new people that weren't on their radar last week when I went in, as they specified I'd made it to this stage and so had six others. From my perspective, if I was the best candidate I don't know why they wouldn't just hire me, and if I wasn't (or if there were a few good ones to choose from) I don't know why they wouldn't just choose. My girlfriend says it's like buying clothes or a new phone or something; just because you found one or two that might be great for you, you're still gonna go in the other stores and keep looking and researching to make sure you're making the right choice. So do you know if this normal and why an employer might keep looking/interviewing for people even after they might have found a suitable person? And is this a bad sign for me? (Am I overthinking?)

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