Error increases as frequency increases using FreqCount Library in Arduino Uno Board

[SumanTheBob]

#include <FreqCount.h>
#include <LiquidCrystal.h>

LiquidCrystal lcd(5, 4, 3, 2, 1, 0);

void setup() {
Serial.begin(9600);
lcd.begin(8, 2);
lcd.print("Freq:");
FreqCount.begin(1000);
}

void loop() {
if (FreqCount.available()) {
unsigned long count = FreqCount.read();
lcd.setCursor(0, 1);
lcd.print(count);
lcd.print(" ");
}
}
I am using this program for measuring the signal frequency of a square wave (8 v p-p) from a very good function generator with frequency varying from 2 kHz to 10 kHz. But the error increases as the frequency increases. (For 2 kHz the LCD output is 2003 Hz, for 5 kHz it is 5006 and for 10 kHz the LCD output is 10011: hence the error is always positive and it increases with frequency) Can you please guide me on why this is happening?

[colinluthier]

Are you feeding the clock of the Arduino with a calibrated source? Replacing the crystal with a TCXO will help with accuracy and stability.
Alternatively you can add a bit of code for an offset for your particular crystal.

[PaulStoffregen]

You say "the error increases as the frequency increases", but to my eyes the error looks like a pretty consistent percentage of the frequency measured.