DC converter use in solar cell MPPT
For long time of searching I found my answer in research paper DC-to-DC Converter With Low Input Current Ripple for Maximum Photovoltaic Power Extraction
From this paper

the dc-to-dc converters must assure continuous input
and output current to provide maximum energy flow from the
source to the load.


The current ripple creates problem in power electronic
systems, which affects the voltage and, thus, the power. The
extent of the loss problem due to PV current ripple is a feature
realized for the first time in this paper. The exponential curve
of the PV cells causes nonlinear mapping in the current axis
which in turns leads to high loss of output power due to
the input current ripple.

This paper also suggest efficient DC converter which have continuous input and output power.  Foranswer

Did I get something wrong?

No, this paper support my idea.

Are there any theories to explain how this works?

It's use LC component to filter input current but still high ripple.

If I was right, how to correct this design?

We can us other type of DC  converter such as SEPIC, Cuk or D type converter