Question: I am using Office Version 2206 (Build 15330.20230), the current version. When I am searching for an email it shows a message that maybe not all results are shown and that it is still indexing 127279 emails, many more emails than I have recieved since the issue occured. When I search for something it only shows results that more than a year old.

This problem occurs only since a couple of days, possibly since the last update from July 06.

Answer: 
The most common reason is because computers use the binary system, where
a bit can be either a zero or one.
If computers used ternary values for the bits, then we'd have everything in powers of 3.

As regarding RAM/memory:

A number N of bits in an address bus (used to select an address)
can address `2^N` bytes.
Whenever the number of address bits increases to `N+1`, automatically
the addressable space increases by a factor of 2.

The manufacturers will naturally use the maximum address capacity
when including memory chips in the design, so memory size will naturally
be in powers of two.

As regarding register sizes:

The same reasoning applies, since internally the hardware may address
a bit in the register using its number, which again is in binary
notation.

(All this is just a supposition and an enormous simplification of the
real situation. I'm sure that an electrical engineer will be able to
demonstrate why circuits based on the binary logic will naturally
use a power-two. As the Intel 8086 has shown, other numbers are possible,
but may be costlier to manufacture.)
Generally, processors are referred to as x-bit, where x is the width of the data bus.

In rough terms, increasing the data bus size offers primarily speed as a reward...twice the bits, twice the data access speed. This is the ideal; some inefficiencies are involved (e.g., fetching 1 byte still takes 1 cycle on a 32-bit system, same as fetching 4 bytes).

As ctrl-alt-delor points out, there are some logistical advantages to stick with a bus width of 2\*\*N. The only restriction of following this pattern is that you have to double the width every time you introduce a wider architecture, giving a max access speed increase of 2.

And in the end, it's hard to justify a new architecture if it's not at least twice as fast as the old one.

Note that low end microcontrollers offer many exceptions to this rule, and the properties of audio and human perception have made 12 bits a popular width for DSPs.