When a part is not exactly the size indicated on a drawing it still may be used if its measures are within the tolerance set by the engineer or designer.
Tolerance is indicated in a + and/or - deviation of the constructer's measure.

Mostly when we use tolerance it has a range in thousands of the unit used. If a bar is measured in 1000 mm the tolerance might be +/- 1 mm in total. Depending on what it is meant to be made for.

For example when a shaft should have a size of 1000 +/-0.5  mm then the deviation may be at most + or - .5 mm meaning 1000.5 or 999.5. If it is more then that the shaft should be reworked until it has the correct size.
If the shaft is for rough iron work, like iron gates or beton works, this tolerance is perfect. But when it is meant for a calculator device this will not do and the tolerance has to be put lower in the ranges say of 1/10000th. So you can inmagine that when mister Babbage wanted this kind of tolerances and his engineers machines could not produce that Babbage tought to have a problem.
There are various methods to calculate the minimal and maximal tolerances and yes for this also there are tables.
So you might see the importance of tolerances combined with tables. If the calculator made a mistake in the table and the contructor would copy the mistake. Quality control would approve the shaft. Off course this shaft will never fit into the hole it is made for. Resulting in loss of manpower, material and time: money.