Can anyone explain why one action results in a trapezoid shape and the other a rectangular shape?

In the attached video, why does Solidworks think it's necessary to extend the length of the first line drawn in the sketch to match the length of the third line drawn when I change the length of the third line draw by entering a larger value in the dimension dialog pop-up box but NOT when I increase the length of the third line drawn by dragging its endpoint?

I was careful NOT to set an EQUAL RELATIONSHIP to the length of the first line segment drawn and the third line segment drawn. Who has control over the RELATIONSHIPS? You? Me? Solidworks? The behavior shouldn't be a haphazard kind-a-thing. Does it seem right to any of you? And if so, please offer an explanation. I'm not asking if you know what's gonna happen in advance of performing either of the procedures I just performed and recorded in the video. I'm pretty sure we all know what will happen now that we've seen it. What I'm asking for is a logical explanation describing why a programmer would assume it is a good idea to do two completely different things when a user changes the length of a sketched line - regardless of how he changes it. To me, this is akin to dipping your brush into a bucket of paint and brushing a white line left-to-right onto your bedroom wall but a red line on a return stroke right-to-left. I can make no sense of it.

In the video clip (GC_Forum_Q1.mp4) at the point where I chose and deleted the HORIZONTAL RELATIONSHIP of the second line drawn, if you choose Display/Delete Relations - All In This Sketch; you'll see 4 that apply to the first, third and fourth lines drawn. None are EQUAL RELATIONSHIPS. And none of the 4 shown apply to the second line drawn. So you can see my dilemma.

2 Answers

Apart from the point that it isn't THAT important, I suppose:

When you add a dimension, it thinks that you're trying to make a rectangle and tries to help.
When you drag the corner, it is sure that you don't want a rectangle and leaves you to draw what you want.

I have to agree with Yahya, it does not matter.

In one case you dragged an end point. In the second case you applied a dimension.
The main difference here is that you have intelligence and were in control when dragging the sketch.
In contrast, applying a dimension left SW in control. SW is free to resolve the sketch as it desires. SOLIDWORKS, like many wonderful things (water, electricity, me) is lazy. SW found it "easier" to maintain the "angle" of the line when resolving the distance constraint. If it did not do that, then it would have to solve two things, the distance mate, AND deciding what angle to place the line at. The path of least resistance won again.

If the angle of the line is important, define it with an angle dimension. Or, draw the line at an angle to start with. Making a box, deleting constraints, then dragging a corner is not much/any faster than drawing 4 lines in the desired shape.