Otherwise known as ‘Gresham’s Law’…
In those times coins were minted by almost everyone who had enough suitable materials and tools so their value was determined by weighing them rather than by their ‘nominal value’. I’m speaking here about the coins made of gold and silver, not about the copper ones. No matter how diligent the minter coins were seldom absolutely equal in size because of the crude technology.
Things were further complicated by the fact that they were minted out of almost pure gold – very easy to process but also prone to wear and tear. On top of this, callous ‘operators’ used to file some of the precious metal from each coin that fell on their hands. That’s why it was very reasonable to hoard the newly minted (good) coins and to let go the used (bad) ones, specially if you were in a position of authority and those who were at the receiving end could not protest your shenanigans.
Altogether a clump of very rational decisions, right? It would have been foolish not to file some gold out of the coins that passed through your hands if you had the opportunity – everybody else did it so why be the only one not to?, it would have been foolish not to keep for yourself the good ones and pass on the bad and it would have been foolish to argue with the power figures.
Only the end result was what Jesus had seen when he entered the Temple.
Technologically the problem was solved by the ‘close’ minting process where the coin is pressed inside a die instead of between a ‘hammer’ and an anvil as before. This way the coins became consistent (weight-wise) and precise (round). Also, after the ‘knurls’ were added any attempt to file them became obvious so people started to trust their face value.
But human greed has no limit so those who used to control the minting process, by now in the hands of kings and such, started to lower the gold content…