TTCalc uses binary floating point numbers. It means that your input values are first converted to a binary representation and then the calculations are performed. After calculating the result is again converted from the binary to the decimal (you can select the input and output format on the display tab). You must remember that not all values can be converted from binary to decimal (and vice versa) without loosing accuracy. For example decimal '5' can be converted to binary '101' and the '101' is exactly equal decimal 5. But decimal '0.3' has not a good binary representation, it is '0.010011001100110011.....'. And when you put decimal '0.3' the calculations are performed on an approximate value and the result is only an approximation too.
For example try to calculate: 0.204 - 0.34*0.80 + 0.068, you would expect that the result was 0 but TTCalc gives you: 3.15544362088404722164691426e-30 which is a good approximation to the real zero (look at e-30 part which means 10^(-30)).