Row greater than max size (8060) but I can't see why

I have a table that has 483 columns. A date not null, a char(6) not null, and then 481 x float sparse null. By my maths, even if every sparse float is non-null, the row should still only be:

-none
date: 3 bytes
char(6): 6 bytes
sparse float: 12 bytes x 481
sparse update overhead: 2 bytes x 483
= 6747 bytes and way below the limit of 8060 
    (even allowing lots for other random overheads)

In fact, the real-life row causing the issue still has 133 of the float columns as nulls and SQL Server claims this row has a row size of 8108 (and is too big). What is the ghost that is taking up my row space?