Sign in/out/up in English correspond to actions you might take in your daily life (eg signing in to a visitors' book at a meeting), whereas most people would only have come across log in within a computer related context.
Many non-native English speakers primarily get in contact with English in computer related contexts since they live in a non-English speaking country and use English language websites.
This group of customers may be larger than the group of native speakers depending on your target audience. So optimizing for non-native English speakers living in non-English speaking countries is something which should be considered.
