why is RC= exactly 63.2% full charge, and how is 5RC considered 100%?
Take a look how a capacitor charges through a resistor: -



Capacitor charging follows this exponential relationship: -



Where Vs is the target "supply" voltage and, if you let t=CR you find that Vc/Vs = 0.632 or 63% for short: -

\$\dfrac{V_C}{V_S} = (1-e^{-1}) = 0.632120558
\$

When you do the math of stacking 5 lots of "63%" on top of each other you get: -

Vc = 0.99326 x Vs or, put another way, 5CR gets you to within 1% of the full charging voltage.

Try this page for more background info.