Sine Wave Generated using Simulate Signal Block with phase 0 & 180 degree doesn't add up to zero?

[vibhutesh]

Both sine wave has same amplitude. I have made the phase of one sine wave to 0 degree & other to 180 degree. Theoretically it should add up to zero, but as visible in the results its having a exponential increase. Although its of order of e-16, which can be considered as zero. But I just wanna know the reason of this appearance.

[ttrr]

It is due to the round-off error that you did not get the exact 0.

http://en.wikipedia.org/wiki/Roundoff_error