Why has SoftBank increased less on the Frankfurt Stock Exchange than on the OTC market?

Living sometimes in Canada and sometimes in France, I use both Questrade and Degiro.

My question relates to the following stocks:

• OTCMKTS: SFTBY (SoftBank on OTC Markets – with Questrade)
• FRA: SFT (SoftBank on Frankfurt Stock Exchange – with Degiro)

I don’t understand why these two stocks didn’t increase as much as each other.
Aren’t they supposed to represent the same asset on the Tokyo Stock Exchange?

From 19 Mar 2020 to 16 Feb 2021

• SFTBY has increased by 285%
• SFT has increased by 229%

This makes a difference of 56%, which seems a lot to me.
How can it be explained?

In your opinion, should the OTC market be preferred to foreign shares on the Frankfurt Stock Exchange? What are the advantages/disadvantages of one or the other?