This is what I've seen in the past 15 years of working in various agile environments. It doesn't matter what the official purpose of sprints is--the effect is that developers are pushed to deliver chunks of functionality in 2 weeks, often that were "estimated" by a group that's composed of developers of widely different levels of experience and thoughtfulness. Even when the developer doing the work is the one doing the estimation, it's impossible to know what it will really take to do the work and very difficult to resist the pressure to over-promise.