ATtiny pin numbers and Software serial

i started playing attiny 85 , want connect via software serial.
reading this tutorial got confused on pin numbering. code:

code: [select]

#include <softwareserial.h>
// definitions
#define rxpin 3
#define txpin 4

softwareserial myserial(rxpin, txpin);

int sensorpin = 7; //actually pin labeled "2" on hlt tutorial
int sensorval = -1;
...

the rx-pis 3 , 4 seems reference pb3 , pb4 (physical pin 2 , 3) sensorpin seems reference physical pin number 7 , not pb2.

why , how know pin numbering system use?

this can confusing.
they using physical pin 7 of chip analog input
so pin can referred a1, can referred pb2.
since using the  pin analog input, should refer a1 pin ide
i have attached file clearer ....
the first time read instructable, found confusing tooo

Arduino Forum > Using Arduino > Microcontrollers > ATtiny pin numbers and Software serial

arduino