Because the older bond has had more time to accrue more interest. Go to the calculator you used to determine the bonds values. Change the year of one of the $50 bonds - try shifting it back a year or two - or more. Watch what happens to the value.

I have 5 saving bonds. One that's face value is 500, bought 12/1994 and currently has a value of 537.80. Then two bought on 12/1998, face value of 50 and, currently has a value of 40.08. And 2 bought in 11/ 1998, face value of 50 and, currently has a value of 36.36. How is it the 1994 bond has interested over its face value yet, the cheaper bonds haven't?