Or rather, that simple math is a simplified version of the truth, which involves internal losses, per charge changes in the structure of the materials in the battery, etc. etc. etc.
The different types of rechargeables have different internal losses, different charging losses, different ways their internal structure changes over time and charging.
Reductionist and proud of it.
Being ignorant is not so much a shame, as being unwilling to learn. Benjamin Franklin
Chase after the truth like all hell and you'll free yourself, even though you never touch its coat tails. Clarence Darrow
A person who won't read has no advantage over one who can't read. Mark Twain