the files listed with du -a do not match ls -R, possible bug?

I think I found what may be a bug in the du program.  So far I have only been 
able to observe this anomaly in my build directory for glibc.  The bug is 
uncovered with the following commands, issued from the base of the build 
directory:
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# ls -lR | grep '\.op$' | awk '{printf("%s\n", $NF)}' | sort > ls_c_files

# du -a . | grep '\.op$' | awk 'BEGIN {FS="[\/]"} {printf("%s\n", $NF)}' | 
sort > du_c_files

# sdiff -s du_c_files ls_c_files
                                                              > mcount.op
                                                              > profil.op
                                                              > sprofil.op
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Thank you for your time and effort.

Aaron