You're correct that they are indeed separate permissions but I think when Google started putting them in buckets for displaying to the user they lumped the two together.

For instance on one address book app the website currently shows them asking for: Contacts/Calendar -read your contacts -modify your contacts

While the app on my phone says: Contacts -read your contacts -modify your contacts

According to Android Police an update earlier this month to the Play Store app appears to have separated them again: http://www.androidpolice.com/2014/12/06/apk-teardown-play-st...