In the US many nouns become verbs by adding -ize (standardize). These same words usually end in -ise in Britain, despite the British dictionaries which show -ize as the main form with -ise as an alternative. One consistency is the American -yze words (analyze) are all -yse in Britain.
Sometimes words in British and American English are identical in meaning but spelled differently, as in sulfur and sulphur, hemoglobin and haemoglobin. Most words (taken from the French) in Britain ending in -our end in -or in the US (color, colour); most words in Britain ending in -tre end in -ter in the US (center, centre). The trend in American spelling is to drop letters that are not needed in a word, such as the ‘u‘ in ‘colour’. The US has a greater tendency to drop silent consonants and vowels, and move to a more phonetic spelling, especially where the old spelling was a French remnant (tyre, tire).
The irregular form is generally more common in British English and the regular form is more common to American English for these verbs: burnt, burned; dreamt, dreamed; learnt, learned; smelt, smelled; spelt, spelled; spilt, spilled; spoilt, spoiled.
Licence and license are both valid in the US, but in Britain the former is the noun, the latter the verb (same for practice and practise). A good dictionary will indicate both American and British spellings when there is a difference.
Why do the differences exist? Well, America chose to differentiate itself from Britain from its beginnings; spelling was included in that. The spelling of such terms at theater instead of theatre and color instead of colour is legacy.
