TI68k: Unexpected integration'questionable accuracy'

nInt((sin(sin(x)))^2 - (sin(cos(x)))^2, x, 0, pi)

returns, in a few seconds, -2.59E-14 and a 'questionable accuracy' warning, which is odd because the integrand is well-behaved with no singularities in the range. Individually integrating the two terms gives no warning, then summing those two integrals gives zero to machine precision.