Damaging Coins

וכן המרקע דינרי חברו, והעביר צורתן--חייב לשלם משום גורם. וכן כל כיוצא באלו הדברים

Here, the Rambam in Chovel UMazzik 7:11 says that if one rubs someone else's coin until the tzurah is erased (thus devaluing it), the halacha it that this has the status of gerama. That is, it is not considered directly damaging the person's property.

The Ktzos HaChoshen (386:11) asks on this Rambam from a different Rambam in Gezailah 3:4 as follows:

או שגזל מטבע ונסדק או פסלו המלך, או שגזל פירות והרקיבו כולן, או יין והחמיץ--הרי זה כמי שגזל כלי ושיברו, ומשלם כשעת הגזילה

Here, the Rambam says that if one steals a coin and it cracks or the king passuls all coins of that particular tzurah it has the status of one who steals a keli and it breaks, where the halacha is that you pay the value of the item at the time of the gezailah.

So, the Ktzos asks, we see from this second Rambam that even if the coin merely cracks or the king passuls it is considered like direct damage. If so, also by one who rubs out the tzurah it should be considered direct damage. Why then does the Rambam say that it is merely considered a gerama?

Update - the answer: [Before I get into the answer to this I would like to strengthen the question slightly. The gemara in Baba Kama 98a brings Rabbah's statement that if one is shaf matbeah (he rubs out the tzurah on a coin) he is patur. Tosafos say that Rabbah must hold that paslaso malchus, if the king is mevatel a particular tzurah, it does not have a din of nisdak (i.e. as if the coins cracked). Tosafos explain that if Rabbah held that paslaso malchus is like nisdak, so forsure rubbing out the coin would be like nisdak and he should surely be chayav for rubbing out the tzurah.
Tosafos is referring to a gemara earlier regarding gezailah where Rav Yehuda says that paslaso malchus is the same as nisdak in regards to the din of paying kshaas hagezailah. Rav Yehuda basically says that just like if one steals a coin and it cracks the thief pays the value at the time of the gezailah, so too by paslaso malchus. Tosafos is saying that if one held like Rav Yehuda, than surely by shaf matbeih one would be chayav. Since Rabbah holds patur he must not hold like Rav Yehuda. So basically tosafos is saying that the two dinim are interrelated. If it's the type of damage that makes a gazlan pay kshaas hagezailah, it should also be the type of damage a mazik would pay for. The question then is, why does the Rambam distinguish.]

There are two basic suggestions that I will make in the shitas haRambam. The first is the suggestion of the ktzos himself. He says that the Rambam distinguishes between shinui and hezek. According to the Rambam shaf matbeah is a shinui and therefore a gazlan would pay k'shaas hagezeilah. However, it is not a hezek and therefore considered only a gerama.
The other way to go here is to say that the Rambam holds that the hezek to the coin is in the category of garmi (a higher form of gerama) and we will have to say that garmi has the status of hezek, enough to make a gazlan pay kshaas hagezailah. This answer is offered by the Even HaEzel Chovel Umazzik 7:4 who discusses this possibility.