What are the variations in resistance?

There is ±30% variation in the value of the series bias resistor (R₁) from the typical value as shown in the datasheet. The value of the base-emitter resistor (R₂) is not specified separately; instead, the resistor ratio (R₁/R₂) is specified. There is ±10% variation in R₁/R₂ from the typical value.
The values of bias resistors are dependent on temperature, as shown in Figure 2. The resistance decreases at a rate of roughly 0.2%/°C.

About resistor R₁:
R₁ converts the voltage applied to the B terminal of a BRT into current. A bipolar transistor is a current-driven device. When a bipolar transistor is voltage-driven, the rate of change of the collector current with respect to voltage becomes large, making it difficult to control the collector current. R₁ in a BRT makes it relatively easier to control the collector current. When a BRT is on, the internal transistor operates in the saturation region where h_FE (=I_C/I_b) is in the range of 10 to 20, depending on the input voltage. Therefore, a relatively large current (I_B) on the order of a few milliamperes flows through R₁. Since the allowable power dissipation of R₁ is 1/8 W, the maximum input voltage (V_I) of BRTs with a high R₁ value is determined by the value of R₁.
(See the FAQ entry “What is the maximum voltage that can be applied to the base of a bias resistor built-in transistor (BRT)?”.)

About the resistor ratio (R₁/R₂)
The input voltage (ON) specification (V_I(ON)) is dependent on R₁/R₂. Since the base current (I_B) does not flow immediately before the transistor turns on, the voltage applied to the B terminal (V_I) is divided by R₁ and R₂. Let the turn-on threshold voltage of the internal transistor be V_be. Then,    
V_be ＝ R₂ / (R₁+R₂) * V_I(ON)
The V_be of BRTs is not affected by the values of R₁ and R₂ if they contain the same transistor.
V_I(ON)＝V_be＊(R₁+R₂) / R2 ＝ V_be＊( 1 + R₁ / R₂ )
Hence, V_I(ON) is dependent on the resistor ratio (R₁/R₂).