How this is calculated. Vout = Vin · R2 / (R1 + R2),
and the current through both resistors is
Vin / (R1 + R2). Two things
catch people out. First,
the divider is not a voltage source: its output impedance is
R1 ∥ R2, so anything you connect draws current through R1 and pulls the output
down. The loading error above shows exactly how much for the input resistance you entered — a
10 kΩ/4.7 kΩ divider into a 1 MΩ ADC input is fine, the same divider into a 10 kΩ input is not.
Second, a divider is a poor way to power anything: it burns current continuously, and the
output moves with the load. Use it for sensing, not supply.
The load is shifting the output by more than 1 %. Lower both resistors to stiffen the divider, or buffer the output with an op-amp follower.