The basic terms are not interchangeable, each is clearly defined, and when people refer to varieties in one breath and hybrids in another, then it is obvious that the speaker is either confused, or is using non-botanical meanings. There are long, internationally defined rules for the naming of species and garden cultivars.
In the wild, species (shorthand sp.) are more or less uniform in habit, foliage, flowers and fruit. Any variation is part of an evolutionary process, and botanists apply subdivisions within a species (sub-species, varietas, and forma) to recognise such differences. All remain more or less stable in the wild, but when grown together in cultivation they may hybridise and the distinctions become blurred. This variation is exploited by gardeners who select (recognise and name) an individual plant, and propagate to maintain it.
A sub-species (shorthand ssp.) is a “mini-species” with distinct morphological or genetic variation, and a sometime distinct geographical distribution. I don't believe that Canna have any defined sub-species, certainly not in Dr Tanaka's revision of the Canna species.
A variety (or varietas, shorthand var.) is a wild variety, and its differences from the species are less clear-cut than a sub-species, e.g. Canna indica var. maculata.
The form (or forma, shorthand f.) is used for colour variations or similar minor differences.
HybridIf several species of one genus are cultivated together, they may hybridise (shorthand x), giving rise to offspring sharing characters of both parents, for example Canna x ehemanni (C. iridiflora x C. indica var. warsczewiczii). Seedlings from these crosses may vary, and may be selected and given cultivar names. If the resulting hybrids are fertile, several generations of plants may be produced. In time, the parentage of the offspring becomes obscured, reflected in the style of names chosen, for example Canna ‘Ehemanni’.
There are also variations, when we refer to F1 and F2 hybrids, but those require a separate explanation as we are then categorising genetic variation.
There is no rocket science about this terminology, it is just a simple and straightforward way of classifying our wild and garden raised plants.