I wouldn't have called them "very different" in the context of the magnitude of the rest of the values.
The (probable) reason you are seeing this difference is that Mathcad Prime and Mathcad 15 use different floating-point libraries and their parsers likely give a different order of calculation (whilst maintaining overall correctness of calculation).
Floating-point arithmetic in Mathcad uses 80-bits internally, with 53 (IIRC) bits for the mantissa, which is sufficient to store 15 to 17 decimal digits. Operations on floating-point numbers will generally produce a result that has more than 53 bits, which consequently get dropped. This is quite normal for "reasonable" values and the user will usually never notice, given Mathcad's default number display. If you go to the Math Formatting tab and select 15 digits (the second item in the "Results" area of the tab), then you are likely to see some other differences in the trailing digits.
The values you are seeing are basically round-off errors caused by the inexact nature of floating-point arithmetic and are close enough to zero to be regarded as zero. This will happen in all maths applications that use standard floating-point arithmetic.
If you want, you could explicitly make each value less than, say, 10^-9 equal to zero, or round everything to 9 digits accuracy.
Stuart
Stuart
