Solve a Digital Mystery at Arlington National Cemetery

I usually
walk on the ground of Arlington National Cemetery, not only for exercise but because it is a place of beauty and peace and comfort. Nearly every visit includes a stop at the Tomb of the Unknown Soldier. Why does the date on the stele on the front of the Arlington Memorial Amphitheater show the exact year it was built – 1915 – as MCMXV, but the cornerstone on the mausoleum side of the amphitheater shows the year as MDCCCCXV . The second format is incorrect, at least in the currently accepted format, but it does say 1915 – following a complicated logic. Why is there a contradiction?