OK, this is a bit surprising, but there is a logical explanation. I was actually surprised to see two digits on the other parts...

The Java lexer looks at the start of the expression. It sees an addition of two ints, so it does that (result is zero on your code). Then it continues, and see a string. It converts the result to a string, do the concatenation and continue with this operation.
I think it is equivalent to doing:
StringBuilder sb = new StringBuilder();
int r = t[5]+t[4];
sb.append(r).append(":").append(t[3]).append(t[4])... and so on
The problem is in Java: automatic coercion of values to stings in concatenation operations, and using the same symbol for concatenation and plus operation...
You found yourself the workaround. Should you want to do an actual arithmetic operation in the middle of the concatenation (sometime we need to do that, often i+1 or similar), you use parentheses:
text((t[5] + t[4]) + ":" + (t[3] + t[2]) + ":" + (t[1] + t[0]), 25, 50);

Thanks for the answer.  It really didn't occur to me that within the context of "text()", the  "+" would do anything but concatenate.  That it did both is somehow more confusing, yet I now feel strangely confident I can use one or the other at will.