I am a lowland nepenthes enthusiast with limited growing room. By far, my favorite nepenthes is N. belli. I love the plant's small stature and tubby pitchers. However, I(along with many others) have noticed a suspicious amount of variabillity among N. belli clones.
I am not a taxonomist, nor have I seen N. belli habitat, however, the "variabillity" of plants being circulated in cultivation now stongly indicated influences from other species rather than variation within a species. Until about a decade ago, N. bellii was uncommon in cultivation (one of my earliest memories of CP's (at age 6, circa 1996) involves a local grower showing me N. bellii pitcher, telling me how incredibly rare N. belli was). Today, many retailers distribute N. belli. The vast majority of these retailers get their plants from Borneo Exotics, who advertise "a mix of 24 clones" for N. belli. A commercial grower advertised on his website that he just got a shipment of plants(including N. bellii) in from Borneo Exotics. I asked his if he had any particularly squat, red clones. He said he had a couple squat, red plants, but that the vast majority of the N.belli plants he got in were "crappy clones" with long, skinny green traps, thin peristomes, and poorly developed wings and "didn't look like N. belli at all". I have seen many photos of N. bellii in collections, in the wild, and in nurseries.
I can divide N. bellii plants that I have seen into 3 distinct "groups"
Fat, Squat, tiny traps, pinkish-red with large, well developed wings, thick peristome: This is, in my opinion, the nicest N. bellii. Traps stay around 1-2 inches and are sometimes wider than they are tall. Clones fitting this description have been described as "like a miniature N. sibuyanensis or N. merrilliana".
Suspiciously Large N. belli: These N. bellii plants are pretty much identical to the previously described "form", but are larger in all respects. Traps reach 4 inches and the entire plant is larger than other N. bellii plants
Narrow, thin peristome N. belli: These have fairly thin traps with a distinct "hip". The peristome is thin. Traps can be green, orange, or pink/red. Strongly resembles a miniature N. x merilliata
Nepenthes bellii occurs sympatrically with N. merilliana and N. alata (maybe N, truncata and N. mindanaoensis but I'm not sure). I have a feeling that many clones of N. belli in cultivation are not "pure" N. bellii, but hybrid swarms. Plants that appear to be "intermediates" between N. bellii and N. merilliana (such as my 2nd example) are strongly suggested to by hybrids and have been observed living in N. bellii habitat for years. Both "giant" N. bellii and "runty" N. merilliana plants exist in cultivation, suggesting hybrid swarming between the sympatric N. bellii and N. merilliana. Likewise, the "skinny" N. bellii plants that have been distributed via borneo exotics strongly resemble miniature N. x merilliata plants, and are most likely IMHO hybrid swarms involving N. bellii and N. alata.
I am not an authority on Fillipino Nepenthes or any Nepenthes for that matter, this is just a hypothesis I have thought up to explain the "questionalbe" N. belli plants floating around in cultivation.