Scratch

Most computers can not represent decimal numbers exactly.  They store a binary approximation internally and show you a rounded-off value in the display routines.  So what is probably happening here is that on the last pass through the loop, the value is something like 4.9999997 so it passes the test, has 0.2 added to it, which is then rounded-off for display and you see it as 5.1.  Decimal number comparison can be quite tricky, some programmers deal with this by programming in a small tolerance (much smaller than the step size but larger than the round-off error) to their comparisons to make sure they work.  So they might compare My Variable to 5-0.001.  This is not a perfect solution and maybe there are people in this forum who have a more elegant approach.

Yes, probably.  Integers are usuaully much safer for comparisons, depending on how they are stored internally.  I don't know how Scratch stores integers, whether it even differentiates between decimal numbers and integer numbers.  Maybe one of the moderators will address this.