Software for any system is complex. And it’s quite common for almost every software project to be late on schedule. The Triple Constraint — “schedule, quality, cost: pick any two” doesn’t even fit software engineering in any kind of serious endeavor because it’s mostly a “pick one” scenario.

If you’ve worked on projects where all these three were met with the initial projections, then whoever is estimating those has really made sure that they’ve added massive buffers on cost and time or the project is too trivial for a one person team to do in a month or two.

The entire reason Agile came up as a methodology was in recognizing that requirements change all the time, and that the “change is the only constant” refrain should be taken in stride to adapt how teams work.