It appears to be based on the strength of the words in the title.
To the HP filters a title like: Colorful Images of Birds is seen as colorful-images-of-birds. Each word is this title is common and each word is assigned a value. If however the title were "Colorful Images of Quetzalcoatl Birds" the same filters would see it as:colorful-images-of-Quetzalcoatl-birds, because at leas one word is rare and its value higher the starting pre-publication score would also be higher. Another factor is how many titles with the same or very similar words have been previously published. Even if the hub has not been published, HP will look at the number of visits for a pre-determinate period of time, and will count your own visits. Leave an unpublished hub alone for some time and you will see the scores start to go down.
At least this is what I have been able to gather after closely monitoring my hubs for three months.