Your thoughts?
To whom it may concern,
I apologize for the issues raised during my background check.
Allow me to make clear that there was never any intent to conceal any part of my employment history. When I realized I had not listed my short term employment with Y and Z, I made sure it was listed for the background check. With my background never being an issue before, I did not realize the questions that could arise and I now understand that I should have also informed <this company> of this additional information.
Again, I apologize for any inconvenience caused by this issue and will answer any questions needed to clarify this situation.
Sincerely
Pixilated