This is a precision issue with datatips. See this answer to learn how to specify the level of precision in datatips.
Demo of method 2 from that link:
x = linspace(0,2*pi,100);
y = sin(x)*.0001 + 37.5;
ax = cla();
h = stairs(ax,x,y);
ax.XAxis.Exponent = 0;
% Modify datatip precision
h.DataTipTemplate.DataTipRows(2).Format = '%.5f'; % Y to 5dp
% Show datatip
datatip(h, h.XData(25), h.YData(25));
datatip(h, h.XData(47), h.YData(47));
