Very old versions of Anki treated reviews like you describe - they were scheduled to a particular second, and it resulted in them becoming available throughout the day. Many people did not like that; they wanted to have access to all of their cards at any time the next day.
Learning steps were initially intended to be used for short intraday periods, so per second scheduling made more sense. When users started using them for multi-day delays, it reintroduced the same issue of reviews trickling in throughout the day, and so the code was changed to treat intervals that cross a day boundary like reviews are treated.