Alexa rank is usually updated on daily basis.
But we have noticed that sometimes the daily updation job falls a little behind. We have raised a ticket with Alexa on this issue. Alexa has acknowledged that it is a problem and they are working on fixing it.
Link to the ticket : https://alexa.zendesk.com/requests/23868
This is the exact reason why you see a horizontal line on your site’s alexa rank history graph sometimes on siterankhistory.com. We will keep publishing such occurrences through this blog.
From 01-Mar-2013 this has happened on the following dates.
Meanwhile we can sit and wait for our latest Alexa ranks!