If I entered my tax withheld information correctly, why is the IRS requesting me provide supporting documentation?

They want to match it to the W2 your employer sent in or maybe they didn't get the employers copy.  Does your 1040 line 16 match your W2 box 2?  Sometimes other payments like estimated payments show up on line 16 by mistake.

Maybe the employer never submitted the W-2's to the SSA with forwards the info to the IRS.   but does the reason really matter!  if you're due a refund your not going to get it until you comply with the IRS request.