There are some words contained in the UK Spellodrome word lists that do not belong to the word family the list is designed to practice (e.g. vice, space, grace appearing in a word lists ‘Suffixes –cious’). There are two reasons why you’ll see words that don’t obviously fit into a list:

1. There are lists of statutory words that students have to learn each year; these are dotted around all of the other word lists so that students' encounter them all throughout the year. The reason for this is that many of them don’t fit within any of the other word lists. 

2. In lists like the ‘Suffixes –cious’ list, root words may have been included to highlight to students how the word changes when it becomes an adjective (see p.18 of the spelling appendix document)