Wiggle in the current source of an emittter follower amplifier
I don't see the answer here, yet, that you should be seeing. (Yes, BJTs vary all over the place, one to another. But that does not answer why LTspice shows you what it did, which uses an exactly-the-same-every-time BJT model.)

There are three important problems (at least) with your simulation:


No accounting for the Early Effect in your thinking or testing.
No accounting for base drive affecting the mirrored collector current.
"Signal" doesn't properly keep the devices in active mode.


So let's draw up a schematic that fixes #2 and #3 so that you can focus on the effects of #1 and see it in action.







simulate this circuit &ndash; Schematic created using CircuitLab

The addition of \$I_1\$ and \$Q_4\$ ensures that there will be a rather constant \$1\:\text{mA}\$ into \$Q_1\$'s collector. \$Q_4\$ is tasked with supplying the base current for both \$Q_1\$ and \$Q_2\$, so that it doesn't subtract from \$Q_1\$'s collector current. And the use of \$I_1\$ gives us a perfect current source, which is better than a resistor because we don't know the collector voltage of \$Q_1\$.

The use of a DC sweep from \$2\:\text{V}\$ to \$8\:\text{V}\$ ensures that \$Q_3\$ isn't either smashed up against the \$10\:\text{V}\$ rail or else smashed down against \$Q_2\$'s collector. So this keeps things on an "even keel" so to speak.

Once this is done, we can now run this:



And here you can now clearly see the Early Effect.