It really reveals to me the inherent limitation of Theme and variation form where you have a primary theme and then write series of variation of that theme.
Now the pros of Theme and variations is that it ensures a sense of thematic unity in the piece. Every variations in the piece are linked to the opening of the piece and hence there is a sense that every variation is part of a greater whole (I certainly rate the form higher that through composed music or the Russian way of composing which is a series of different themes)
However the issue is, how exactly do we link these variations together?
well the simple way is to have no linkage at all and to treat each variations as an individual "numbers" (in an operatic sense) like the Goldberg Variations/Diabelli Variations where after the variation, there is a break and pause. So essentially the composition is like an album and each individual variations are an individual tracks of the album.
However what happen if you want to create seamless continuous music?
Well Beethoven created an answer to this question in the 5th Symphony 2nd movement which is a double variation where each individual variation, there is a gradual increased in intensity. Therefore there is a sense of continuous narrative flow in the piece that sounds perfectly coherent and has no sense of abruptness at all.
However what happen if you want each variations to contrast with each other (aka in a similar way as the two theme in the exposition of the sonata form contrast with each other)?
If you do what Liszt did, simply writing each variation to be completely different to each other. The results is you have a discontinuous piece of music and although each section are connected thematically, one variation doesn't seamlessly join and connect to another variations. You create music that is abrupt and really essentially a medley of variation and the only thing linking them together is the underlying theme and although there is a connection there isn't the same continuous flow you will see in other competing forms like a sonata form movement and as Greenberg famously said "any hack can write a medley"
This reveals the inherent limitations of theme and variation form and part of the reason why I am skeptical about Ode to Joy final movement of Beethoven Symphony No. 9. Where there is a similar theme and variation with abruptness between each variation due to the high degree of contrast within each variations.
However within the same "Ode To Joy" movement despite it flaws, Beethoven created a solution to this conundrum. In between the military march variations and the Choral entrance variation that are highly contrasting, there was a transitional section/fugato section linking those two variation together. Which in my opinion is the most brilliant part of the symphony. Beethoven solved that problem of Theme and Variations in that Symphony but unfortunately he didn't do that for the entire movement.
I hence proposed to codified that "solution" that Beethoven created with a form called "Theme And Variations and Development" form.
Where every single variations has a transitional section linking each variation together. Hence no matter how much contrast each variation has there is always a transition section that has developmental qualities linking each variation together.