Before diving into topology specifics it would help you to understand modes of switch mode power supplies (SMPS's). There are a lot of great resources out there, one of them is Colorado boulders online Power Electronics Specialisation course. It's free, and I think pretty amazing. It helped me immensely, as, I self studied to get into this industry.
Definitely take that course, and, all your questions, initially, will be answered. If for some reason you don't feel like that, key words you can look up are Continuous Conduction Mode (CCM), and Discontinuous Conduction Mode (DCM), boundary (Or critical) Conduction Mode (CrM or BCM). Look these terms up, and reply back here which you think it is and why, we can go from there.
But, because I'm new to this I have to give an answer here...
CCM is what you are seeing because the instantaneous current never reaches zero as you point out. The instantaneous current is continuously above zero, meaning, during the switching cycle, Ton (switch ON time) and Toff (switch OFF time), the instantaneous current does not touch zero. That's the basic idea of CCM.
CrM, is where the instantaneous current touches zero, but almost immediately increases back to some non-zero level. So, goes from zero to some non-zero value, never staying at zero.
DCM, is where the instantaneous current stays at zero for some amount of time.
They each have there pro's and con's, different modes work practically better for different topologies and control types. But, these questions are answered by long books. Hopefully that helps