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.)
I think some of the arguments missing is that it is convenient that a register can be divided into two parts which are easy to work with, for instance with smaller registers. Hence, for the intel architecture, you initially had 16 bits registers that were subdivided into two 8-bit subregisters, and then these registers were extended to 32 bits, keeping the least significant digits addressable as 16 bits register, and then to 64 bits. If you had a 48 bits register, it would leave an awkward 16 most significant bits part, and you would have to play with 16 bits and 32 bits subregisters... What a pain !

Moreover, when you multiply the contents of two registers, you need a register of twice that size (or two registers of the initial size) to contain the result. Having registers that cannot be combined two by two would make things complicated for arithmetic.

I guess this view is supported by the fact that, for addressing (and not arithmetic), the processors internally use other numbers of bits.