Diff side by side output with "|" (incomplete) sign

When I compare two files with just numbers, in addition to < and > signs, I also have | signs at some of the lines. For example,

25746 | 26123

I checked the manual and it says

‘|’

The corresponding lines differ, and they are either both complete or both incomplete.

This statement is not fully clear to me. I checked such lines and couldn't spot a difference than the others. How can I fix these lines to lines with < and >?

Thanks!

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