There is a reason.
The reason is, that the way the number is stored in memory, in case of the double data type, doesn't allow for an exact representation of the number 3600.2. It also doesn't allow for an exact representation of the number 0.2.
0.2 has an infinite representation in binary. If You want to store it in memory or processor registers, to perform some calculations, some number close to 0.2 with finite representation is stored instead. It may not be apparent if You run code like this.
double num = (0.2 - 0.0);
This is because in this case, all binary digits available for representing numbers in double data type are used to represent the fractional part of the number (there is only the fractional part) and the precision is higher. If You store the number 3600.2 in an object of type double, some digits are used to represent the integer part - 3600 and there is less digits representing fractional part. The precision is lower and fractional part that is in fact stored in memory differs from 0.2 enough, that it becomes apparent after conversion from double to string