This is going to be a rather pedantic and slightly scientific post! Monofilament tippet materials fall into two groups: nylon copolymer and fluorocarbon (Polyvinylidene fluoride, PVDF). There is often quite a bit of debate in fly fishing around the properties of each and their respective advantages when it comes to actually catching trout. However, a little digging on the physical properties of fluorocarbons suggests that much of the fluorocarbon monofilament used in fly fishing is made by the Japanese Kureha Corporation, under the brand name Seaguar. Kureha publish some useful data on their fluorocarbons and a simple comparison between Seaguar fluorocarbon and polyamide copolymer (nylon copolymer) reveals some basic differences in density, refractive index tensile strength and tensile modulus. The first two of these properties are often cited as advantages in using fluorocarbon monofilament, i.e. its higher density means it sinks faster and its refractive index slightly closer to that of water makes it less noticeable by the trout. However, anglers often have mixed views on the relative strengths of the two materials often feeling that fluorocarbons can be more susceptible to breakages.
The table lists some of the basic physical properties of both nylon copolymer and fluorocarbon monofilaments, so a monofilament with a tensile strength of 100 MPa takes twice the force (Newtons per square millimetre) to break than one with a tensile strength of 50 MPa. If you then make two identical diameter monofilaments from that material, and don’t significantly change the mechanical properties of the material in doing so (a bit of an assumption here), one will still take twice as many newtons per square millimetre to break. In terms of basic tensile properties (strength and modulus) the differences between nylon copolymer and fluorocarbon are not huge, though possibly still significant when it comes to a sudden shock on your leader. However, it seems that processing of polymers can indeed change some of their physical properties. PVDF, in particular, has five phases  with specific properties relevant to specific manufactured products, e.g. β-phase for ferroelectric applications, γ for heat-treated, etc.
It would seem that it is therefore entirely feasible that differences in manufacturing processes could have a significant influence on final mechanical properties of fluorocarbon so perhaps not surprising that different brands from different sources of ostensibly the same product have different properties and different experiences for the angler. All very interesting scientifically, if rather pedantic, and probably not of any real concern to the trout!!
El Mohajir, B.-E.; Heymans, N. Changes in structural and mechanical behaviour of PVDF with processing and thermomechanical treatments. 1. Change in structure. Polymer 2001, 42, 5661–5667