Resistors and other electronic parts are rated to be within a certain percent of the specified value. A 1000 ohm resistor with a ±10% tolerance could have an actual value between 900 and 1100 ohms. So what’s the actual range of values in a bunch of resistors? Due to manufacturing techniques, it might not be what you expect:
A 10% carbon-composition resistor is made in a somewhat slipshod manner. The manufacturer tries to get it right, but some of the variables are just too difficult to control. They make up a batch, test them all, and then throw away the bad ones. What’s left is a distribution of values truncated on either side at the ±10% limits. The other main feature of the distribution is the big gap-toothed section in the middle. That’s where they pulled out all the good parts and sold them at a higher price with a ±5% tolerance. How else do you think they make 5% resistors?
The actual value of a 1000 ohm 10% resistor is usually between 900-950 ohms or 1050-1100 ohms. There will probably be none in the 950-1050 range, they were removed and sold as 5% resistors.