What SHA1withRSA actually does

I had some Java code generating a signature like this:

Signature signature = Signature.getInstance("SHA1withRSA");
signature.initSign(privateKey);
signature.update(plainText);
return signature.sign();

Natively I assumed that the reverse operation would simply be decrypting the returned data to reveal a hash. However the result was a 35 byte string rather than the 40 I was expecting. Turns out the the signature is also ASN encoded:

irb(main):041:0> key.public_decrypt(signature)
=> "0!0\t\x06\x05+\x0E\x03\x02\x1A\x05\x00\x04\x14\xF7\x1C\xB4o&\xCFA\xFFN\x14\xE9\xA4V\x89\xC5K\xC7\xB8\fg"
irb(main):042:0> OpenSSL::ASN1.decode(key.public_decrypt(signature))
=> #<OpenSSL::ASN1::Sequence:0x007f2c9937f718 @tag=16, @value=[#<OpenSSL::ASN1::Sequence:0x007f2c9937f920 @tag=16, @value=[#<OpenSSL::ASN1::ObjectId:0x007f2c9937f9c0 @tag=6, @value="SHA1", @tagging=nil, @tag_class=:UNIVERSAL, @infinite_length=false>, #<OpenSSL::ASN1::Null:0x007f2c9937f948 @tag=5, @value=nil, @tagging=nil, @tag_class=:UNIVERSAL, @infinite_length=false>], @tagging=nil, @tag_class=:UNIVERSAL, @infinite_length=false>, #<OpenSSL::ASN1::OctetString:0x007f2c9937f740 @tag=4, @value="\xF7\x1C\xB4o&\xCFA\xFFN\x14\xE9\xA4V\x89\xC5K\xC7\xB8\fg", @tagging=nil, @tag_class=:UNIVERSAL, @infinite_length=false>], @tagging=nil, @tag_class=:UNIVERSAL, @infinite_length=false>