You are right. If you would go about how Android is usually set up, you would only get about ... 50 Mb or so. Here is how PB set it up:
First, they divide the internal SDcard into 3 partitions: 1 Fat32 and 2 ext2 ones.
Next, they transferred part of the Data partition (including apps) to one of the Ext2 one, giving 380 Mb of free space to install more apps.
Don't forget, before updating, backup you apps! Tutorial here
Posted from my Pocketbook IQ (using Dolphin Browser)