Quake 1: frame timer

If you check Quake 1 source code (sys_linux.c), you can see something like this:

double          time, oldtime, newtime;

double Sys_FloatTime (void)
{
  struct timeval tp;
  struct timezone tzp; 
  static int      secbase; // new epoch

  gettimeofday(&tp, &tzp);  

  if (!secbase)
  {
    secbase = tp.tv_sec;
    return tp.tv_usec/1000000.0;
  }

  return (tp.tv_sec - secbase) + tp.tv_usec/1000000.0;
}

while (1)
{
  // find time spent rendering last frame
  newtime = Sys_FloatTime ();
  time = newtime - oldtime;

  if (time > sys_ticrate.value*2) // 50 ms * 2
    oldtime = newtime;
  else
    oldtime += time;

  Host_Frame (time);
}

Why does Quake do = some time and += at other times? I suppose it must be some floating point issue, but Sys_FloatTime() returns a double, which is very accurate. It is also adjusted to return seconds from the start of the program, it doesn't count from 1/1/1970. Changing the code to use assignment at all times, doesn't seem to change anything.

EDIT: If you check QW source code, you will see, that Sys_FloatTime() is renamed to Sys_DoubleTime(). But even if a float was returned, why would using += vs = make any difference? IMO, ever using a float for something like this was wrong, plain and simple.