Technically, the whole country could just switch to Euros without even being in the EU.
Granted, the only two countries to do so (Montenegro and Kosovo) never bothered with creating their own currency to begin with, they went straight to Euros post-independence (with some disgruntlement from the EU). And then there's also two (Bulgaria and Bosnia) which are technically not using Euros, but their currencies are pegged to Euros and stupidly simple to convert (1 EUR = 1.95583 BAM/BGN, so just multiply/divide by 2).
Likely to make it simple for retailers to accept euro. Making it exactly two means the retailers would lose money on the exchange spread. Another example is with 1 hkd = 0.97 mop.
B/c the conversion to Euro happened later. It was already fixed after the hyper inflation in 1997.
The initital rate was 1000 levs = 1 DM, in 1999 there was a denomination of 1000 : 1, same year the Euro happened and the Deutsche mark enjoyed the same rate to the Euro.
Granted, the only two countries to do so (Montenegro and Kosovo) never bothered with creating their own currency to begin with, they went straight to Euros post-independence (with some disgruntlement from the EU). And then there's also two (Bulgaria and Bosnia) which are technically not using Euros, but their currencies are pegged to Euros and stupidly simple to convert (1 EUR = 1.95583 BAM/BGN, so just multiply/divide by 2).