Alphabet combo changed and score drastically dropped

[Balfourl Winnettm]

Hello, I am playing n=5 for a month. Recently, I downloaded a different version of n-back with a completely different alphabetical set (without t and s) and observed a decrease in score immediately.

That worries me a bit since I thought working memory trained based on n-back should be transferable.

Both alphabetical sets are in English

Any thought on it?