Sloppy Jalopy manufacure an iconic range of figures and vehicles for the WW1 & Interwar 'Pulp Era' and for post 1960 Cold War gaming in 28mm scale.
Sloppy Jalopy's philosophy is that vehicles should `look right` with the wargames figures you use on your miniature wargames table, hence the difference in scales. Great War and Inter-War (or Pulp Era) figures nominally developed for 28mm gaming actually tend to be on the large size, so fit better with 1/48th scale vehicles. Other manufacturers have released vehicles for this period but they tend to look too small. Likewise for the post colonial period (the second half of the twentieth century) the figures commercially available tend to be nearer to a true 28mm size, smaller than the earlier period, so they decided to scale the vehicles for this period to a 1/56th scale.