Welcome to the world of floating point numbers, where not all numbers can be represented exactly in binary form.
See this thread for more information - https://in.mathworks.com/matlabcentral/answers/57444-why-is-0-3-0-2-0-1-not-equal-to-zero?s_tid=faqs_link
When comparing floating point numbers, the best practice is to use a tolerance -
pix = 0.001;
a = -1:pix:1;
tol = 1e-6;
idx = find(abs(a - pix) < tol)
%check
a(idx)
k = ismembertol(a, pix);
IDX = find(k)