Compare those two API requests:

http://demo.piwik.org/index.php?period=day&date=today&showColumns=nb_visits%2Cvisits_evolution&filter_limit=50&sortOrderColumn=nb_visits&filter_sort_column=nb_visits&module=API&token_auth=anonymous&format=json&method=MultiSites.getAll&language=de&

[{"label":"Piwik Forums","nb_visits":898,"visits_evolution":"153,7%","idsite":7},{"label":"Crowdfunding","nb_visits":53,"visits_evolution":"65,6%","idsite":32},{"label":"virtual-drums.com","nb_visits":28,"visits_evolution":"-22,2%","idsite":3}]

http://demo.piwik.org/index.php?period=range&date=last7&showColumns=nb_visits%2Cvisits_evolution&filter_limit=50&sortOrderColumn=nb_visits&filter_sort_column=nb_visits&module=API&token_auth=anonymous&format=json&method=MultiSites.getAll&language=de&

[{"label":"Piwik Forums","nb_visits":5368,"idsite":7},{"label":"Crowdfunding","nb_visits":395,"idsite":32},{"label":"virtual-drums.com","nb_visits":275,"idsite":3}]

The second call does not contain the evolution metric. The only difference is the date and period parameter.

Background: I'm using this API call in Piwik Mobile 2 to fetch the list of all websites for the All Websites Dashboard. I'm displaying only the number of visits and evolution there. To reduce the amount of transferred data and to improve performance I'm using the showColumns parameter.