As suggested, using old material or improper construction/closure of knots can certainly be factors. However, I'm convinced that the commercial extrusion process used to create knotless leaders inevitably results in a tippet section that does not match the test of equivalent tippet material--regardless of manufacturer. I often use commercial leaders for the butt/hinge section of my trout leaders, but I never trust or use the tippets that are incorporated into them.
Whenever I prepare to replace a butt/hinge section, I start by grasping the last two feet or so (basically the tippet section) of the new leader between my hands and giving it a steady pull. In my experience--at least with most leaders 4X or finer--the tippet will almost always break well below the stated test. I have been doing this for so long that it has become an unnecessary step, but I do it anyway.
After verifying that the strength of the tippet is no better (or worse) than all the other extruded leaders I have used, I cut the tippet section off and attach a fresh tippet of the desired length. The method used to attach the tippet to the leader and the leader to the line is up to you, but a loop-to-loop attachment is probably the most efficient and economical for most applications. Not only does it facilitate quick changes of tippet (or leader, if needed), but the butt/hinge sections generally last for most of the season without being eaten up by tippet changes.
There are a number of interesting older threads about knots and/or leaders on this site. Some of these discuss leader designs and even ideas about matching leaders to line (the "flex test," for example). You might want to do a site search for some of those threads.
I'll add one last note about an aspect of knots/leaders in which I seem to differ from some fly fishers:
For most of my trout fishing, I'm not at all interested in the search for "100%" tippet-to-fly knots. Although I want a knot that reliably retains most of the tippet strength, I also want that knot to be the weakest link in the chain. In my opinion, using 100% tippet-to-fly knots often creates a situation in which the knot that attaches the fly is stronger than the knot that attaches the tippet to the leader. (This is especially true if the leader-to-tippet knot is a blood knot, surgeon's knot/loop, or perfection loop). This often results in a full length of tippet material remaining with the fly after a break. Even if reliable 100% knots could be used for both connections, a break would happen somewhere along the length of the tippet. If any part of my leader has to break, accidentally or on purpose, I want the break to be at the fly.
PS--Floatant on the tippet can cause a knot to slip and should never be used to lubricate knots. Water is probably best for that purpose.