Assertion:
There appears to be different default interior padding values controlled by the MARGINHEIGHT and MARGINWIDTH attributes. In other words, if only one of these attributes is specified, the default for the unspecified attribute will be different than the default for the attribute if BOTH attributes are specified. [That looks confusing even to ME =)] It is possible (though unverified) that in instances where only one value is present it is used as the margin value for BOTH dimensions.

Simple IFRAME, no MARGINHEIGHT or MARGINWIDTH attributes applied:
filler text filler text

Simple IFRAME, MARGINWIDTH=20; no MARGINHEIGHT specified:
filler text filler text

Simple IFRAME, MARGINWIDTH=20, MARGINHEIGHT=20:
filler text filler text