CosmosDb ordering by datetime - high RU

Is there a reason why ordering by a datetime property takes a much higher amount of RU than other properties? However it appears the penalty is only paid on the first page of data. e.g. in one of our collections I get these results:

SELECT * FROM c where c.WorkspaceId = 'bayside' and c._type = 'Asset' order by c.CreatedAt

first page: 94.53 RUs (100 items returned, 200 items read)
next page: 11.83 RUs (100 items returned, 100 items read)

SELECT * FROM c where c.WorkspaceId = 'bayside' and c._type = 'Asset' order by c.NameLowerCase

first page: 23.48 RUs (100 items returned, 200 items read)
next page: 11.62 RUs (100 items returned, 100 items read)

SELECT * FROM c where c.WorkspaceId = 'bayside' and c._type = 'Asset' order by c.id

first page: 33.44 RUs (100 items returned, 200 items read)
next page: 14.6 RUs (100 items returned, 100 items read)

The datetimes are stored as ISO 8601.