There's another ためし that fits here. Etymology from Shogakukan's 国語大辞典:
Definitions and pitch accent from Daijirin:
ためし  【例・様】
This has the same pitch accent as 試し, so verbally and in kana, you'll just have to know both terms and figure it out from context.
Since ためし in the sample sentence would mean "past instance of something", and since there aren't any past instances of 理解できた, a better translation might be:
That is not surprising. I
do not always understand have never understood her myself.