Why are there two versions of Sassoon fonts?

In the Sassoon Primary families the alternate font characters are: G I J f k q 4 9 (+ Welsh W w Y y umlaut and dieresis).

The first batch of fonts listed are shown as SINGLE font versions, which means all alternate letters are within the same font. To use them, type all your text in the document, spell check it, then perform a 'find & replace'. For example FIND & REPLACE WITH Alt+0 135. Education users say they prefer this method rather than the conventional way. We made these specially for Schools.

The conventional way is the second batch of font names in the list is shown as TWO-FONT VERSIONS, meaning the alternate letters are held in a separate font. So not only must you Find & Replace, but also format your finds with another font. The advantage is that more alternative letters can placed in the separate font. They are: G I J b f (3) k (3) q r 1 4 7 9