Why do Android phone manufacturers describe RAM in powers of 1,000 but not 1,024 (as PCs do)? For PC RAMs, when a manufacturer say they produce a 4GB memory bank, they are producing a memory bank with 4,294,967,296 bytes (4 GiB). The same applies to CPU cache, i.e. a CPU with 2MB L3 cache means 2,097,152 bytes (2 MiB) of L3 cache.

For hard drives, flash drives (USB drives, SSDs, eMMCs and UFSs) and optical discs (CDs, DVDs and BDs), manufacturers use powers of 103 for B, KB, MB and GB, so a 4GB USB drive has roughly 4,000,000,000 bytes.

According to my observation, Android phone manufaturers use the latter kind of units when advertising RAM. That does not sound reasonable to me because PCs' RAMs aren't advertised that way.

Edit: I am an experienced Android user. I have investigated the kernel and made sure that other components' sharing RAM does not affect my observation.
tl:dr:  Android memory is more in line with hard disk memory and uses the SI or decimal standards unlike PC memory. Standards support this



Basing my answer on What is the advantage to using a factor of 1024 instead of 1000 for disk size units? - Software Engineering Stack Exchange and bringing out only the key aspects. Details can be referred in that question


In the case of PC's, quoting  from the accepted answer binary addressing was adopted



  It is kind of convenience and optimization for getting the most usable, addressable space between the two considerations of addressing and physical platter size



For Disk drives SI units were adopted. From Bobson's answer



  The disk drive industry followed a different pattern. Industry practice, more thoroughly documented at Timeline of binary prefixes
   and continuing today, is to specify hard drives using SI prefixes and symbols in their SI or "decimal" interpretation. Unlike binary-addressed computer main memory, there is nothing in a disk drive that influences it to have a total capacity easily expressed using a power of 1024.


(Emphasis Supplied)

That explains in brief the reasons for binary and SI



Evolution of Mobile Phones and Standards


The earliest commercial model phone  DynaTAC 8000Xappeared on the scene in  1983. It couldn't do anything more than basic calls so RAM doesn't come into play
By 1987 hard drives were marked in SI units as can be seen from the binary timeline linked 
Feature Phones: IBM Simon was the first feature phone in 1994, which had 1 MB of RAM and 1MB of storage expandable to 2 MB - you read that right :) So, when the feature phone cane out , SI standards were in vogue


So, likely reasons :


SI standards were automatically applied to mobile memory as they were more viewed as storage elements rather than computational requirements
Fact that memory was / is not based on platters (see accepted answer ) ,  adopting binary system made no sense . Instead of having separate nomenclature for storage and RAM, SI units were adopted for both. But in the case of PC's legacy continued, though one could argue that it is incorrect. Mac OS since 
has since version 10.6 (Mountain Lion in 2011) shown hard drive size using decimal prefixes and continues to do so
By 1998



  IEC     introduces unambiguous prefixes for binary multiples (KiB, MiB, GiB etc.), reserving kB, MB, GB and so on for their decimal sense



By 2001 Linux Kernel (used in Android) adopted IEC standards