The flip side of the coin

The following is a coin that was minted during the begining of the bar Kochba revolt in 133 ce:



For those who do not know, the full name of Bar Kochba was Shimeon ben Kosiba. He led a revolt against the Romans which lasted for 3 years during which he controlled a significant part of Yehuda (although he never controlled Yerushalaim). The revolt was an initial success but it came to a grinding halt with Bar Kochba's death in Beitar.

Much attention has been paid to the messianic component of the revolt. Rabbi Akiva was the spiritual leader of the rebellion and, along with the majority of the sages of his generation, considered Bar Kochba to be the Mashiach.

What I find interesting about the coins from this era is that the consistently refer to Bar Kochba as "Nasi" instead of what I would have expected which would have been "Melech." The title of Nasi, as far as I knew until I saw these coins was generally reserved for the head of the high court during the time of chazal. I find it surprising that Bar Kochba would refer to himself as "Nasi." Does anyone have any explanation for this?