Sequential testing will never be truly random, unless you can guarantee that your traffic is uniform across time.

Imagine you're running an AB test on a website, where the number of customers you get increases linearly over time. If you test two features sequentially, there is no duration you could pick that is small enough to where you would not expect the second option to win.

The traffic to a restaurant isn't uniformly distributed over an hour window.

There's a peak 30-min in the morning just before work starts for most people, for example.