TV screen size is measured diagonally. That means from the upper left to the bottom right. The measurement will always be larger than measuring across the top and sides of the TV enclosure. Also, some screens have a bit of a bezel covering the edges, so the actual screen measurement might be larger than you can measure with the bezel in place.
Bottom line: your 48" wide TV is easily a 55" diagonal measured display. I just pulled a random Samsung 55" TV on their web page and checked the specs. The TV measures 48.4 inches across, almost exactly what you measured.
Dan
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