I Savings Bonds Interest Calculation Or Formula Can Someone Please Explain This Savings Bond Calculation?

Can someone please explain this savings bond calculation? - i savings bonds interest calculation or formula

I got a series EE savings bonds in the amount of $ 100.00 in 07/1991. I have two different online calculators to estimate the amount of bonds is my punishment now) (18 years later. This is the response I received from two places:

Series EE
Title: 100
The question of the month: 7
Release Year: 1991
Current value: $ 128.92
Interest expense: $ 78.92
Price: 4.00%
Yield: 5.33%
Next Interest Date: 1/1/2010
Final Maturity Date: 7/1/2021

As the link is worth $ 78.92 in interest, but only worth $ 128.92? It seems that won my $ 100 savings bond only a total of $ 28.92 over 18 years (IMO, it's pathetic). Shall I do not understand the simulator?

Thank you.