Most of the other goodybags have been given extra data for the same price in order to make them better value.
But always-on is a difficult one to price correctly. Because it doesn't have a specified data limit. Phones have evolved over giffgaff's lifetime and data-hogging services such as videos have become more popular -- together meaning that the cost to giffgaff of providing that data continuously increases. Consequently giffgaff has to consider increasing its price occasionally -- not doing so would be the equivalent of slowly committing suicide.
The reality is that most people only use such vast amounts of data through laziness. Videos, which can use data something like 30 times faster than well-written ordinary web sites, are best downloaded in advance via a landline or free wifi and can then be saved and watched as many times as you like -- all without using any mobile data, If most people did that, there would be little if any need for an always-on goodybag. Probably most people could be paying £10 instead of £25. And everybody including themselves would be getting faster data speeds when it really matters, due to less network congestion.