why is this? I thought long or double consonant sounds in Hungarian were usually spelled with a double consonant letter.
Usually, yes, but not always: there are some other examples of intervocalic consonant lengthening, but in most cases such pronunciations are considered non-standard or dialectal:
szalag > pron. sza
llag (ribbon)
papír > pron. pa
ppír (paper)
eső > e
sső (rain)
óvatosan > óvato
ssan (carefully)
Such lengthened pronunciations were sometimes fixed in the spelling of surnames:
dalos > Da
llos (singer)
kolár (Slavic word meaning "wheelmaker") > Ko
llár
.
There's another example which is different because the lengthened consonant is not between vowels and where it is standard pronunciation:
"egy" (one) is pronounced "e
ggy"