The terms eCPM and eRPM mean exactly the same, the only difference is the context they are used for.
When a publisher discusses an advertising campaign with an advertiser, the advertiser thinks of the total amount on the invoice as a cost. Therefore the advertiser will think in terms of eCPM. For the publisher sending the invoice, the same amount is a source of income or revenue. Therefore, the publisher could think in terms of eRPM.
But essentially they are talking about the same numbers. Since salespeople usually try to talk the same language as their customers, publishers usually just talk about eCPM when they discuss things with their advertisers, and that’s why everyone in the ad industry started using eCPM as the main term.