Numeric values take at least 5 bytes: starting with 4 bytes of metadata (defining the numeric type) followed by as many as necessary bytes of the integer representation of the number (with the decimal dot out of the picture).
select cast ( CAST (126 as NUMERIC(10,4)) as varbinary(10))
It's easy to see here that 0A defines 10, 04 defines "4" in the type definition, not sure what following "0001" stand for, must be the data type itself.
Following 4 bytes are the binary representation of the corresponding integer value in reverse order.
Reversing 0xE0391300 --> 001339E0,
select CAST (0x001339E0 as int)
Placing the dot after 4 digits from the right gives the expected result.
Zero bytes at the end of the binary representation of a numeric value are optional:
SELECT CAST(0x0A040001E03913 AS NUMERIC(10,4))
Still returns the expected result 126.0000