For example, citric acid has three acidic protons, all of which are carboxylic acids. Despite being part of the same functional groups, they all have very different pKas. Why (and how?) is this?


Question: How much does the build number of an Android OS matter?
Answer: How much does the build number of an Android OS matter?

As a developer, I have never used it for anything significant (other than maybe analytics).
Between the Vendor and Google, it does correlate to a branch/release of the AOSP platform. 

Your other question on OS 4.3 vs 4.4 and what are they key differences?
Search "Android 4.4 vs Android 4.3 feature list". It is quite significant. But as a general rule, each release typically comes with new API's for developers, improved security, and improved efficiency.