You need to learn about critical sections.
What is probably happening is that the variables are being changed by the interrupt routines mid-way through the calculations. Your 'fix' reduces the time spent doing the calculation with the volatile variables thus making it less likely that there is a collision.
What you should do is copy the volatile variables to local variables with interrupts disabled for that brief period.
cli();
int l = encoderLpos;
int r = encoderRpos;
sei();
Because the Arduino is an 8 bit CPU it takes multiple assembly instructions to perform mathematical operations on 16 bit values. Floating point is even worse using many many instructions for a simple addition. Division and multiplication use considerably more. An interrupt has plenty of opportunity to fire during that list of instructions. By doing an assignment like that and then using the new local variables in your calculations the instructions needed to deal with the volatile variables is kept to an absolute minimum. By turning off interrupts during the assignment you guarantee that the variables can't ever be changed while you are using them. This snippet of code is called a critical section.