I was in the technical publishing business (flight and maintenance manuals) in another life. Tables are used as an alternative to graphical plot presentations in flight manuals, and make it a little easier to access information, say, just sitting with the book in your lap. With the graphical plot, you have to unfold the sheet and have it laid out flat so you can work across the page, maybe needing things like straight edges to help locate data points relative to the margin scales on a graph. Mathematic formulae are not normally presented because then that would require pilots to do math, and most pilots go catatonic when forced to do math.
Breaking the information into tables shortens the process of sifting the data you need compared to a graphical plot, but in order to save publishing space, the increments of the data presentation are limited to fairly large increments. A table could be presented that included every unit of information, but to do so might require the table to extend across 20 pages instead of 2. Limiting the table to, say, 10 unit increments, cuts way back on the space it uses, but it requires you to interpolate, or make an educated guess, on the precise unit within that 10 unit block to use. When tables are used like that it's because the interpolated "guess" is accurate enough for the job.
The most extreme example of this might be found in Quick Reference Handbooks used by flight crews for procedures and performance data. The original information for some performance parameter will always be in the Airplane Flight Manual, in the form of a multi-element graphical chart (the root certification document basically), but you normally don't have that handy in the cockpit and if you did you couldn't really use it anyway; you have the QRH.
You are typically using the QRH sitting in the cockpit with a little map light to see, and a graphical plot format just won't work. The QRH condenses the data down to tables, making it much faster to work out the data you need, and when using the tables, interpolating values between listed increments is accurate enough to do the job.
So it's perfectly fine to see a table that gives a result of 10, 20, 30, and 40 say, and you make a guess that the value you need is between 10 and 20, and 17 "looks about right". The idea is that if the true value is 16 or 18, it makes little material difference in the real world in that instance.
It reminds me of when the use of electronic flight calculators to replace circular slide rules became popular in the 80s. Everybody I knew was buying those new fangled flight calculators, but I never bothered because I was cheap, and because the super precise result it gave was not something you could achieve in normal flying anyway. The Jeppesen circular slide rule may only be accurate to a knot or degree, whereas the electronic calculator goes down to decimal places, but nobody can actually fly with that precision and the slide rule is good enough if you're not trying to navigate to the Moon. Plus, if you get good with the Jepps circular slide rule, you can hold it and do solutions with one hand, getting a useable result in way less time than the calculator took with all the key punching.
Using a table in place of a precise formula or a graphical plot is kind of like that.