What it makes really hard to be well on estimating time: Programmers often have to solve problems they solved never before. You rarely solve exact the same prob

What it makes really hard to be well on estimating time: Programmers often have to solve problems they solved never before. You rarely solve exact the same problem multiple times. And what it makes even more difficult: The environments and tools you have to solve these problems are changing. So you don't really have that much experience you can rely of.